1) The 360 version will have content that is more compressed (specifically, movies and audio) or it will have a lot of discs (5 I think?).
2) PS3 has a some padding there to help with load times, and those 3GB file sizes for zone and sys1&2 will be considerably smaller on 360.
Either way, it still ends up being around 4 or 5 discs on 360, which means it's more likely that the content (video and audio) will be compressed more. I'm not sure how this will affect the final product (if at all) but it'll be interesting to see the differences in that specific content (FMV cutscenes).
Yeah, 5 DL-DVD 360 discs just for FMV alone... probably not a great idea on a couple levels. And it's not like 720p downsamples of the CGI & in-engine renders would be the end of the world.
It will certainly be interesting to get a file structure dump even after they reveal the number of discs.