I use 7zip with all the settings maxed out. Some games barely compress. Others go from 15GB to 4GB. It's all about whether the game is using precompressed files or not.
3.3GB - 2.1GB - 3D Dot Game Heros [BLES00875]
6.0GB - 4.4GB - Alone in the Dark - Inferno
8.1GB - 4.7GB - Assassins Creed [BLUS30089]
38.0GB - 37.6 - GB Final Fantasy 13 [MRTC00003] (Mostly Compressed Movie Files)
15.5GB - 7.6GB - Folklore [BCUS98147].7z
4.9GB - 2.5GB - Ratchet Clank - Quest For Booty [BCES00301]
6.3GB - 2.6GB - Sims 3
That's just a few. As for playing compressed files, don't count on it. That was PSP only. At least for now.
Never confuse heroism with simple self interest.
You must first be illusioned, before you can be disillusioned.
Hope is the first step on the road to disappointment.
Winning Lotto Numbers: 6 14 16 22 28 (56)