From the original TS3 launcher thread:2. The Launcher is a slob. If you thought the TS2 package installer was horrendous, with strewing crappily-named files all over your directory, the TS3 Launcher is worse. Not only does it not name or organize the files in anything approaching a helpful manner (sort by set? Creator? Download time? What are those?), but it jams all of the files into single large files in the "DCCache" folder, and strews additional, completely useless copies of files inside the "DCBackup" folder, which it never uses or cleans up. They aren't named anything useful, so you'll have no idea what the hell they do. You can delete this crap. If you ever attempt to DELETE any content, the Launcher will not, in fact, actually give you your disk space back, instead leaving you with a 50 MB file containing absolutely nothing. SOMETIMES it might let you have your disk space back....but usually not.
I don't know how much disk space has been swallowed. Check the "DCCache" and "DCBackup" folders.