DESCRIPTION ELECTRIC MOTOR FOR COMPRESSOR, COMPRESSOR, AND REFRIGERATION CYCLE DEVICE Field [0001] The present invention relates to an electric motor for a compressor contained in a compressor, a compressor, and a refrigeration cycle device. Background [0002] A conventional compressor described in Patent Literature 1 includes an airtight container, a compression element placed in the airtight container, and an electric motor that drives the compression element via a rotating shaft. The electric motor has a stator, and a rotor placed on an inner side of the stator and providing a rotational force to the rotating shaft. An eccentric portion is provided on the rotating shaft passing through the compression element. Citation List Patent Literature [0003] Patent Literature 1: Japanese Patent Application Laid-open No. 2012-231586 Summary Technical Problem [0004] In the conventional electric motor described in Patent Literature 1, the stack thickness of a rotor core is formed to be equal to that of a stator core or the stack thickness of the rotor core is formed to be larger than that of the stator core. In a case where an electric motor having such a relationship between the stack thickness of the rotor core and the stack thickness of the stator core is used in a compressor, flexure caused by a centrifugal force that acts on the eccentric portion provided on the rotating shaft occurs when the rotor rotates, and the axis center of the rotor core is inclined with respect to the axis center of the stator core due to the flexure of the rotating shaft, which hinders an outer periphery portion of an end of the rotor core in the axial direction from facing an inner periphery portion of an end of the stator core in the axial direction. If there is a non-facing surface being a part where the rotor core and the stator core do not face each other in the rotor core, magnetic fluxes emitted from the non-facing surface among magnetic fluxes generated in the rotor become leakage fluxes without being taken in the end of the stator core in the axial direction and a torque corresponding to the leakage fluxes is not produced, which results in a problem that the electric motor efficiency is reduced. [0005] It is conceivable to provide a balance weight to the rotor as a measure to prevent the flexure of the rotating shaft. However, the cost increases when a balance weight is used, and thus there is a problem that the conventional electric motor described in Patent Literature 1 cannot address a need to prevent a reduction in the electric motor efficiency caused by leakage fluxes without increasing the cost. [0006] The present invention has been achieved in view of the above problems, and an object of the present invention is to provide an electric motor for a compressor that improves the electric motor efficiency without increasing the cost. Solution to Problem [0007] To solve the above problems and achieve the object an electric motor for a compressor contained in the compressor according to the present invention includes: an annular stator core formed by stacking a plurality of electromagnetic steel sheets; a rotor core provided on an inner side of the stator core and formed by stacking a plurality of electromagnetic steel sheets; a rotating shaft having one end side inserted into the rotor core; and an eccentric portion provided on another end side of the rotating shaft and placed in a compression mechanism contained in the compressor. A length from a center of the rotor core in an axial direction of the rotor core to an end face of the rotor core in the axial direction of the rotor core is shorter than a length from a center of the stator core in an axial direction of the stator core to an end face of the stator core in the axial direction of the stator core. Advantageous Effects of Invention [0008] The present invention has an effect where it is possible to improve the electric motor efficiency without increasing the cost. Brief Description of Drawings [0009] FIG. 1 is a sectional view of a compressor containing an electric motor for a compressor according to an embodiment of the present invention. FIG. 2 is a sectional view of the electric motor illustrated in FIG. 1 as viewed in a direction of arrows A-A. FIG. 3 is a side view illustrating a state of a rotor core, a stator core, and a rotating shaft in a case where an operation of the electric motor illustrated in FIG. 1 is stopped. FIG. 4 is a side view illustrating a state of the rotor core, the stator core, and the rotating shaft in a case where the electric motor illustrated in FIG. 3 is in a normal operation. FIG. 5 is a side view illustrating a state of a rotor core, a stator core, and a rotating shaft in a case where an operation of a conventional electric motor is stopped. FIG. 6 is a side view illustrating a state of the rotor core, the stator core, and the rotating shaft in a case where the conventional electric motor illustrated in FIG. 5 is in a normal operation. FIG. 7 is a diagram illustrating flows of magnetic fluxes when the electric motor according to the embodiment of the present invention operates at a lower speed and a higher torque than in a normal operation. FIG. 8 is a diagram illustrating flows of magnetic fluxes in a case where the conventional electric motor illustrated in FIG. 5 operates at a lower speed and a higher torque than in a normal operation. FIG. 9 is a configuration diagram of a refrigeration cycle device having a compressor according to the embodiment of the present invention mounted thereon. Description of Embodiments [0010] An electric motor for a compressor, a compressor, and a refrigeration cycle device according to the present invention will be explained in detail below with reference to the accompanying drawings. The present invention is not limited to the embodiments. [0011] Embodiment. FIG. 1 is a sectional view of a compressor containing an electric motor for a compressor according to an embodiment of the present invention. A compressor 100 has an accumulator 51 that separates a refrigerant gas supplied from an evaporator (not illustrated) and a remaining liguid refrigerant from each other, a compression mechanism 60 that is installed in an airtight container 55 and compresses a refrigerant gas supplied from the accumulator 51 via an intake pipe 52, and an electric motor 4 being an electric motor for a compressor, which is installed in the airtight container 55 and drives the compression mechanism 60. [0012] The electric motor 4 includes a stator 3 and a rotor 1. The stator 3 includes: a stator core 31 that is formed by stacking a plurality of core pieces that are stamped out from an electromagnetic-steel-sheet base material being an electromagnetic steel sheet and that is held on an inner periphery portion of the airtight container 55; an insulating portion 32; and a winding wire 33 that is wound on the stator core 31 by a concentrated winding method or a distributed winding method with the insulating portion 32 interposed therebetween and that is supplied with power from a glass terminal 53 fixed to the airtight container 55. The rotor 1 includes a rotor core 11 that is formed by stacking a plurality of core pieces stamped out from an electromagnetic-steel-sheet base material and that is placed on an inner side of the stator core 31 with a clearance 5 interposed therebetween, and a plurality of permanent magnets 12 embedded in the rotor core 11. [0013] The permanent magnets 12 are fixed to the rotor core 11. As examples of a fixing method of the permanent magnets 12, methods such as press-fitting the permanent magnets 12 into magnet insertion holes formed in the rotor core 11 or applying an adhesion on the permanent magnets 12 and then inserting the permanent magnets 12 into the magnet insertion holes formed in the rotor core 11 are conceivable. In a general rotor core, end plates that prevent permanent magnets from slipping out in the axial direction are placed on one end face and the other end face in the axial direction. However, no end plate is used in the electric motor 4 according to the present embodiment. While the length of the permanent magnets 12 in the axial direction is equal to the stack thickness of the rotor core 11 in the illustrated example, the length of the permanent magnets 12 in the axial direction may be shorter or longer than the stack thickness of the rotor core 11. The stack thickness of the rotor core 11 is the thickness of a stack of the core pieces for the rotor core 11, which are stamped out from an electromagnetic-steel-sheet base material. The permanent magnets 12 may be either ferrite magnets or rare-earth magnets. [0014] The compression mechanism 60 includes: a cylinder 62 that is fixed to an inner part of the airtight container 55 and to which a refrigerant gas is supplied from the accumulator 51; an upper frame 65 to which the other end side of a rotating shaft 2 is inserted to close one end of the cylinder 62 in the axial direction; an upper discharge muffler 61 attached to the upper frame 65; a lower frame 64 that closes the other end of the cylinder 62 in the axial direction; a lower discharge muffler 63 attached to the lower frame 64; and a piston 66 that is placed in the cylinder 62 and performs eccentric-rotational movement in the cylinder 62 along with an eccentric portion 2a of the rotating shaft 2. [0015] One end side of the rotating shaft 2 is inserted into the rotor core 11 and the other end side of the rotating shaft 2 passes through an inner space of the cylinder 62 to be held by the upper frame 65 and the lower frame 64 in a state capable of rotating. The eccentric portion 2a that compresses a refrigerant along with the piston 66 is formed on the other end side of the rotating shaft 2. [0016] An operation of the compressor 100 when the electric motor 4 is rotating will be described. A refrigerant gas supplied from the accumulator 51 is taken in the cylinder 62 through the intake pipe 52 fixed to the airtight container 55, and the rotor 1 is rotated by energization of an inverter (not illustrated), so that the eccentric portion 2a provided on the rotating shaft 2 rotates in the cylinder 62 and accordingly the piston 66 compresses the refrigerant. A compressed high-temperature refrigerant passes through the upper discharge muffler 61 and the lower discharge muffler 63, then moves upward in the airtight container 55 through the clearance 5 between the rotor 1 and the stator 3, and is supplied to a high-pressure side of a refrigeration cycle through a discharge pipe 54 provided in the airtight container 55. [0017] FIG. 2 is a sectional view of the electric motor illustrated in FIG. 1 as viewed in a direction of arrows A-A. The stator core 31 includes an annular back yoke 31a, and a plurality of teeth 31b provided on an inner periphery portion of the back yoke 31a at regular intervals in a rotational direction and extending toward the center of the back yoke 31a. The "rotational direction" is a direction of rotation of the rotor 1. Each of the teeth 31b includes a winding portion 31bl that is fixed to the back yoke 31a and on which the winding wire 33 is wound, and an end portion 31b2 that is formed on an end side of the winding portion 31bl and has a rotor facing surface 31b3 formed in an umbrella shape. A plurality of slots 31c that are each defined by the back yoke 31a and the teeth 31b and that house therein the winding wire 33 are formed on the stator core 31 thus configured. While nine teeth 31b are formed on the back yoke 31a in the illustrated example, the number of the teeth 31b is not limited thereto. [0018] A plurality of magnet insertion holes 11a provided at regular intervals in the rotational direction to correspond to the number of magnetic poles, and a rotating-shaft insertion hole lib that is formed at the center of the rotor core 11 in the radial direction and to which the rotating shaft 2 is inserted are formed on the rotor core 11. A sectional shape of the magnet insertion holes 11a is a rectangle with a width in the radial direction narrower than a width in the rotational direction, and the magnet insertion holes 11a extend in the axial direction and pass through the rotor core 11 from one end to the other end. Permanent magnets 12 with polarities different from each other are inserted into adjacent ones of the magnet insertion holes 11a, and the permanent magnets 12 respectively inserted into the magnet insertion holes 11a have flat plate shapes alternately magnetized with N poles and S poles. In the electric motor 4 thus configured, the winding wire 33 is energized with a current at a frequency synchronized with an instructed number of rotations, whereby a rotating magnetic field is generated and the rotor 1 rotates. While six permanent magnets 12 are inserted into the rotor core 11 in the illustrated example, the number of the permanent magnets 12 is not limited to that in the illustrated example. Furthermore, while an IPM (Interior Permanent Magnet) electric motor 4 is used for the compressor 100 in the present embodiment, an electric motor of a type other than the IPM type may be used for the compressor 100. [0019] FIG. 3 is a side view illustrating a state of the rotor core, the stator core, and the rotating shaft in a case where the operation of the electric motor illustrated in FIG. 1 is stopped. In FIG. 3, illustrations of the insulating portion 32 and the winding wire 33 among the constituent elements of the electric motor 4 illustrated in FIG. 1 are omitted. FIG. 3 illustrates a relationship between a stack thickness Lr of the rotor core 11 and a stack thickness Ls of the stator core 31, where the stack thickness Lr of the rotor core 11 is the thickness of a stack of a plurality of core pieces for the rotor core 11, stamped out from an electromagnetic-steel-sheet base material and the stack thickness Ls of the stator core 31 is the thickness of a stack of a plurality of core pieces for the stator core 31, stamped out from an electromagnetic-steel-sheet base material. The stack thickness Lr of the rotor core 11 is formed to be smaller than the stack thickness Ls of the stator core 31. One end face lldl of the rotor core 11 is located on a side nearer the center of the rotor core 11 in the axial direction than a position on an extension line 31b41 of one end face 31b4 of the stator core 31, and the other end face lld2 of the rotor core 11 is located on a side nearer the center of the rotor core 11 in the axial direction than a position on an extension line 31b51 of the other end face 31b5 of the stator core 31. That is, assuming that the length from the center of the rotor core 11 to an end face of the rotor core 11 is Ll and the length from the center of the stator core 31 to an end face of the stator core is L2, L2 is formed to be longer than Ll in the rotor core 11 and the stator core 31. When the rotor 1 is not rotating, the position of an axis center 11-1 of the rotor core 11 coincides with the position of an axis center 31-1 of the stator core 31. [0020] FIG. 4 is a side view illustrating a state of the rotor core, the stator core, and the rotating shaft in a case where the electric motor illustrated in FIG. 3 is in a normal operation. Upon rotation of the rotor 1, flexure caused by a centrifugal force A that acts on the eccentric portion 2a occurs in the rotating shaft 2, and the axis center 11-1 of the rotor core 11 is inclined with respect to the axis center 31-1 of the stator core 31 due to the flexure of the rotating shaft 2. Because the stack thickness Lr of the rotor core 11 and the stack thickness Ls of the stator core 31 have a relationship Lr