It may be the sound. MGS4's disk was maxed out not because of the cutscenes which is a popular misconception but because of the uncompressed sound.
Not sure if this is the case with GoW3, but I can't imagine this type of game being so long as to warrant that kind of space. The graphics are incredible but so they are in UC2 and KZ2 and neither of them, to my knowledge utilized this amount of space.