It is unknown so far whether this is from better compression or less files actually being installed, but it is known that, after the update, you will need to delete and re-install games.
No concrete answer yet on whether loading speed increases result.
Some numbers quoted from GamersReports.com:
Call of Duty 4 - 0.2gb smaller
Call of Duty: W@W - 0.1gb smaller
Gears of War 2 - 0.4gb smaller
Halo 3 - 0.6gb smaller
Battlefield: Bad Company - 1.6gb smaller
Street Figher IV - 0.4gb smaller
Left 4 Dead - 1.3gb smaller.