According to Pete Hines, VP of PR and Marketing at Bethesda, the efficiency is due to the team’s new engine that they used for Skyrim, and new optimization techniques that they have developed specifically for the game.
“6 gigs: New engine & we’re much better at compression (art/voice/data). It’s huge yet more optimized than we’ve had before. And faster,” he said on Twitter.
EGM’s TAKE: Hopefully these techniques can be improved even further in the future so that bigger HDDs are not required.