I did a 30-second video for AH once. Not counting the exported actual final product, all the working files, the sounds, the separate clips, the saved editing files, etc.... When compressed together they took up over 1GB of space. My "Soviet Tragedy" one ran 2.5GB I think. Even on a 100GB HD you run out of space fast.
I did a fresh install of Win7 on a 100GB drive and after installing AH and 1 other game (figure maybe 10GB max for both?) I was down to 50GB free.
Everything takes its toll.
I have a 1 TB drive with only 250GB left free on it. That will probably clear up as I go through and clean out old program and game files, etc, but a lot of the things I do with videos and just watching other videos, all take up tons of space.
Not to mention the skin files. Oi! and film files!! I have over 7GB of files in my AH directory and all its sub folders (before I backed them up to start fresh on the new system).
Steam games go way out of their way to use as much HD space as possible, too.
You'll need more space when you need it. When you need it you get something that will last a long time. That's why folks are getting 1TB, 1.5TB, and 2TB drives. So they don't HAVE to get another in 6 months.