Swoop, if you have 100MB of film files, it grows. If you have skins enabled, it grows a LOT. If you have high-res and skin packs installed, it grows a LOT MORE!
Then there's the maps. There's a bajillion crappy-arsed HTH maps that do nothing but take up HD space (that's why I think duh's total is so large, personally)
Mine tops out at 3.04GB, but that's because I do a lot of skins. I keep all the iterations of my skins, all the work files, all the archives, under subdirectories in the skins directory, so they're all added to this total, as well.
203MB main dir
~100mb terrains
770MB cache
32mb films
1.91GB skins (that's mostly my skin files)
40MB sounds (sound pack)
EDIT: My skins files/archives come out to about 1.7GB or so, I just checked