Well that's assuming they were even planning on using anywhere near that much before the decision to go dual platform (or multi, if it's on the PC).
And the thing is that content is not always about capacity, no matter what some claim (this included developers).
Graphics are unlikely to be cut, since the system's are close in power anyway.
FMVs are likely to take up the most space. If there is any cutting to make an easier fit on the 360, it would likely be there. Or they would have higher resolution ones on the PS3 version, as GTA IV did.
Another thing to cut would be audio. Not necessarily removing tracks, but not using uncompressed audio tracks in the PS3 version, assuming both versions are meant to be identical.
But in terms of proper game content, I wouldn't worry about that. You can still fit a hell of a lot on even one disc. Plenty of 360 games have shown that. So fitting loads of contend on three to five (if it comes to that) will still give you plenty of game.