He made it with AAA assets, and didn't do much work to size them down. Plus the the core of the engine, UnrealEngine4 is heavy on its own. Not really meant for lil indie games, so its kinda big.
Alright could this game transformed to mobile , it's nice and unique , I love Tower defense games once I start playing with I can't stop playing it , so did my nephew did since it's easy to understand (and has lazy work through )