yeah DS on PS3 wasn't even that big. I'm pretty sure Dark is bigger in size than DS but nothing major. You can do open world games i n2 discs, you'd have to duplicate everything except story CGI/voice etc for each disc. The actual levels themselves don't take up that much space.
Well, the game was made into one open world, so there won't be many loading screens. If anything, the 360's frame rate might drop, or menus might be slow. Happy that I'm getting both ps3 and 360 versions. (Ps3 for co-op, xbox for pvp)