Sorry to hear about your storage problem. The only thing I can think is going on is the game is producing an enormous log file because of some repeating error. However this would take quite a bit of time to produce a file that consumed that much. Moreover, every time you launch the game a new log file is started, freeing any space it previously used. Anyway have a look in this folder ~/.config/unity3d/Denki/Autonauts/Player.log and keep an eye on it as you run the game.
Failing that, I really have no idea why it would use that much.