I remember when this happened in WoL, around a year after release. Blizz pushed a big patch, 'optimizing' the game and reducing the overall folder size down to a manageable 6.5 gb or so.
I literally can't install SC2 again until LotV is optimized. 19.5 gb is an INSANE amount of space for a game that isn't World of Warcraft. It isn't like I even want to run the game on anything but minimum settings either! I only ever use min settings anyway!