Go-Pro file sizes?

 I shot some video the other week with my Go-pro hero 4 black, in fact I shot about 15 gb -ish, not too bad you'd think.... Except when I convert it in Go-Pro studio it ends up around 35 gb, + the original 15gb, then I have to export it, thats another 3.5 gb (and thats only 720p)
 So my original 15 gb is now around 54 gb.
 God help if I start shooting full length films in 4K, I'll be using a cray supercomputer just to store the bloody footage....