GTA 4 DLC pay $20 for episodes that give you 10 hours extra gameplay.

[Rock_on_2008]

All the extra content could easily be put on a PS3 Blu-Ray disc that has up to 8 layers, each layer holds 25Gb. So potentially 200GB on one PS3 Blu-ray game disc.
360 though using DVD's is limited to 9GB's per game disc.