I was confused too because games like this usually are 25 to even 70 gs, but you will see that the graphics are REALLY optimized and the game doesn't require you to download so much data.

It's 15gig uncompressed, I guess the requirements are not entirely accurate but that's what Activision has listed! Either way you're getting the full game, we don't chop up the game. The downloads are handled by Steam servers.