Add mods into the equation, and you can have 30-40GB games. Worse still, most of these games are actually compressed already when on the hard drive, they need further decompression at run time (hence the insane load times on some titles). Then the icing on the cake is as Altrus says, MyDocuments for save games, temp files, config, screenshots, movies, patches, downloads for DLC, etc, etc. Just because the game directory is 21GB, doesn’t mean the entire game install is 21Gb, lol.
Anyone that’s played Unreal Tournament will know the pain of mods and the massive file sizes that can come with them. Few years back with UT 2003/4, I had over 30GB of mods tucked away, the game install directory was something like 68GB at one point (most of it uncompressed duplicates). Although the worst offenders are Beta’s. If you ever get invited to a closed beta, make sure you have a lot of free space, while I was part of one, I had a download directory 120GB, install of 19GB and 5GB of temporary, not to mention 100’s of screenshots and fraps video’s.