so, even if they did leave the textures as they were originally made for the ps3 which would use more space, the 360 couldn't handle the processing of the textures. and if they downgraded the textures any more, it would look like a game from the last generation. so to compensate, they downgraded the textures as much as they could without harming the graphics to an easily visible degree. and the downgraded textures and videos were still just a little to big for 2 discs so they had no choice but to use 3.
first, i am not trying to start any fanboy wars. i read in a few places where square had to redo some of the textures and fmv's in lower resolutions for the xbox 360 version of the game because of the 360's hardware.