The PS3 version of FF13 is 38 GB (6.8 GB for in-game content, rest for full motion video and 7.1 channel audio). In Xbox 360 DVDs (6.8 GB per disc), that's 5.6 DVDs (round up, that would be 6).
But considering that the Xbox 360 can't do native 1080p (or at least that's what I've heard. It can only upscale to 1080p) and it can't do 7.1 channel audio, they could have easily fit FF13 on 4 DVDs with the same in-game texture quality as the PS3 (720p) and the same assets (ie. foilage and what not). With 720p FMVs and 5.1 channel audio being the only downgrades (since that's the most the 360 can do FMV and audio wise).
Honestly I think downgrading the game's in-game visuals to 576p is a lot worse (for the 360's and SE's image) than adding one more disc to the collection so that you can keep the in-game visual quality intact (with only the FMVs and audio being sacrificied because the 360 can't do 1080p video and 7.1 audio anyway).The average gamer is going to care more about graphics than the number of DVDs.
With this whole situation with Square-Enix in general, you have to wonder how relevant Square-Enix is as a technological innovator. What does it say about Square-Enix as a company when Feel+ (who handled graphics and coding while Mistwalker did design) made the better looking jrpg (Lost Odyssey) on the Xbox 360? Square-Enix cut corners because they didn't want to pay more money to Microsoft for the extra disc but in the long run, they would have made more revenue if the 360 version looked better than it looks like now.
And then there's the whole, "we can't make towns in jrpgs anymore! It'll take 10+ years" thing. Whereas Feel+ managed to do so just fine.
And Square-Enix really bombed with The Last Remnant development. They don't even know how to use Unreal Engine 3.0 properly when many previous western AND Japanese developers have figured it out before (including Feel+ and the small Korean developer who did MagnaCarta II). SE couldn't port TLR to the PS3 in time for the scheduled simultaneous multiplat release because they didn't know how to use Unreal Engine 3.0 properly on the PS3. What a joke.