Two reasons:
- Console hardware & software are very specifically designed & integrated. That PS3 game is written for exactly 1 level of detail, at exactly 1 resolution (720p), and exploit various approved & unapproved tricks specific to the PS3 chips.
- The native resolution of the 27" iMac is 1440p.. much larger than any PS3 game has to support. The PS3 technically supports 1080p, but isn't powerful enough to actually run games well at that resolution, so no serious 3D games exist for it.
If you were to take a specific resolution (such as 720p) and compare the 21" 650M and 27" 660M, the 660M would definitely perform better. But I'm talking about running at the resolution that is native to the display - 1080p for the 21" and 1440p for the 27". Running at resolutions other than native results in either a letterbox effect or some blurring from scaling, neither of which are exactly desirable.