It's odd that people would believe that multi-platforming a game would result in more sales.
NGS on PS3 had over 500000 copies sold exclusif to the PS3.
DMC4 had 500000 copies sold on both system combined.
And DMC4 was released 1 year after NGS, and there were a stronger install base, on both system...
In Final Fantasy's case, they sell a minimum 7 million, always has (except for FFIX and FFXII).
So why would they need to multi-platform there game?
They don't know how much time it would take to make it on X360 nor how many DVDs it would take in order to play the game.
Considering that LO took 4 DVDs for barely 10s of hour of gameplay, Final Fantasy would easily take 7 DVDs, can you imagine the production cost and how it would play on the X360?
Enter a town, change disc, exit, change disc......
I easily see Final Fantasy XIII take up 40GB-70GB+.
If a shooter like Resistance 2 would take a full 50GB BD, then I can easily see Final Fantasy XIII take up much more than that.