Because the Xbox360 can't handle the graphics PS3 is capable of, the games that run on both consoles are much simpeler for PS3.
Let's take Call of Duty: Black ops as example.
Black ops is so compressed, that it could fit more than 10 times on a blu ray disc.
PS3 only games like Killzone 3, would need SEVEN XBOX DISCS to have the same specifications as PS3!
This is very disappointing in my opinion.