Keep in mind that despite its incredible level of graphical sophistication FF12 only filled up 1/2 of a regular DVD. So by filling up a Dual-Layer DVD there is 4x the space that was used.

So if we are talking 4 disks (which FF8 was), there is 16x the space that FF12 took up, which is the ballpark, I as an outside observer who doesn't know anything about game encoding, would estimate it to be.

It’s all about how efficiently they code it. (I'm so going to get burned for saying this...) A game like FF12 used 1/2 a DVD, whereas Star Ocean 3 used 2 DVDs. And it certainly appears as if FF12 is a noticeable more graphically advanced game.