GTA IV for the 360 doesnt have scaled down textures
The reason the PC release is so big is because it is multi-lingual, so it has all the audio tracks present, for the 360 (not sure about PS3), you have different language releases, so it all fits on one DVD
Whats interesting with the PC release is that it is obviously cheaper to do one version with an extra disc, than more than one version with only one disc.