Forza 4 and GoW 3 are both on new XGD3 formated DVDs that give the developers about 1GB of extra space. That's why Forza 4 takes up 7,6GB when it's installed and it also makes the drive spin for about 1-2 mins even when the game is installed.
MS said that some drives, particular the older ones from the pre HDMI era may have issues with these discs. IIRC they even promised replacements for gamers that have said versions of the drive/360.