We don't really know what the total size of the textures is or what they are scaled for. They might be scaled up to and beyond 1080p (1920x1080) for the PC version.
For the 360, they might just scale the whole thing to run at 720p and use compression. This would lower the space needed dramatically. Some of that 30GB will be audio as well. We don't know how Funcom is storing the data for the PC version, so it's hard to tell.
I could see it being 2 DVD's. One that installs stuff, taking up 5-10GB, the other DVD the game runs off of. You only use the install DVD once, using the "Play" DVD from then on.