Unreal Tournament was 6GB compressed. Next Generation games are going to be 20GB plus, and how we're going to fit them on DVD9's I don't know, they'll probably be a few of them. On the PS3, we're going to be using the majority of the space on those Blu-ray disks. So, online isn't really the best option in some instances.
Source: http://www.computerandvideogames.com/news/news_story.php?id=125925
Could this be a X360 bottleneck?