I think the two biggest culprits would be sound (all language tracks on one disc, a HUGE advantage of the Blu-Ray) and probably putting the full game on the disc two times to make for a seamless experience without having to resort to the "installation" issues that other Ps3 games have.
I doubt there is actually 50GB worth of pure game on this disc, so don't go thinking like that. Can you imagine....Oblivion and expansions were only like 7-8GB....that would be a HUGE game.
@ Lone_Canus_Lupus, print of the full game twice on a disc allows the machine to read from the exact same data two times in one disc rotation.
I'm sure it requires a little programming to pull of to the maximum extent, but it is the equivalent of cutting the disc read time in (about) 1/2.
~This rant and more from the singing game geek!
