Does your rhetoric make you happy? You realize that MGS4 only took up a whole bluray because they burned up the extra space with uncompressed audio right?
Why does everyone keep on saying this? Has it ever been confirmed, or is it all just speculation?
After all, the Dolby Digital (a form of audio compression) logo appears when you start up the game.
Whatever you think, the game has a lot of textures (there must be several hundred for the Octocamo alone) and a lot of spoken dialogue (as much as, if not more than, most JRPGs, which are shipping on 3 or 4 discs on the 360). It's not hard to imagine that MGS4 would take up several of the 360's DVD9s, even with loads of compression.