I'm going to second what jetman said. I would check the game data utility. A lot of games install files to the hard drive for faster loading. You may be able to delete stuff there for games you haven't played recently to free up some space.I don't even really know what's filling mine up. I wish the PS3/4 had a nice little layout of the HDD I could look at for easy cleaning.
Also, I'm fairly certain I can start preloading the USF4 upgrade DLC on PS3 tonight. If I can, I'll let you know how big it is.