Hm, now that I think of it, the game is rather oddly huge. Not as though these are super hi-res HD sprites with thousands of animation frames. And it isn't exactly bursting with sound effects and music tracks either. A game like Skullgirls, which literally has hundreds-thousands of hi-res sprites per character is only around 6-7 GB...
I get the feeling there's probably a lot of junk data in the game's files, which is why it can compress 4+ GB down to less than 150 MB. There's simply not a whole lot of "real data" in the uncompressed files.