I got a $5 steam copy in the last sale, and I'm planning on picking up a second copy from GOG to play instead.
The steam version is just really messily thrown together, all the director's edition "extras" thrown in a folder deep in the game's steamapps directory, every voice over language file included by default, and all required to be downloaded and remain installed as part of the game. It requires 14GB, largely because of extras and unused files. I don't think much effort was put into packaging it up for Steam at all.
It's 10 or 11 Gigs even without the languages, as I recall.
I own a physical copy and a Steam copy, but I might get it on GOG too, just to support the devs and see it on my GOG shelf. This is my all-time favorite RPG.