Well, if I remember correctly, it is not a true 5 GB install. But the game saves stuff on your HD as you find it so that it does not have to load from disc. This helps so there are no load times in the MASSIVE world as everything loads from the HD as opposed to off disc.
I don't think there is a way around it though. In the end it is near 5 GB on your drive from all the stuff you come across.
BELOW - Yeah, I was thinking the same thing. I know it doesn't install off the top, so I am sure you could still play it.