They may fill up some more storage space with DVD/HDTV quality cut scenes and music, but other than that - it's not going to be actual 'game worth' that will fill up that space. Games are generally 12-30 hours long (excluding online games and epic-RPGs like FF) with some actually creeping down to 8 hours long. Code is relatively small, storage wise - it is media that takes space.
So to fill up those blueray disks, they'll probably just add a lot more videos to the game. It's just me - but videos don't make a game, especially if they're stopping me from playing (i.e. unskippable cut scenes).
(Yes, there may be a relatively mild increase in texture sizes, or numbers of textures, but otherwise it's not going to be that much extra needed space).