When they're programming the game and all that, they have meetings to discuss a target for the game's requirements (CPU, GFX, HD Space, etc)
So imagine, they're all sat round the table.
"Well Jenkins, have you finished programming the textures and interfaces?"
"Yes, a very small problem, it's 70 gigs+".
-Silence-
"Carry on!"
I mean come on, myself on my modest 160gb HD has around 25gigs left, maybe 30 if I cleared up some stuff I didn't use. 70gigs means new HD time.....