I think that uninstalling the game can help. Another solution would be to delete files from the mentioned directory, when you will not be able to start the game, it will mean that you have deleted too many files.
Besides, 15 GB is not much, I have a basic game, plus two dlc and my directory has almost 25 GB.
Apart from removing files added in the last patch, eg a one and a half hour movie in hd, there is probably no way to size down the game directory.