I can't remember exaclty, but I'm quuiiiite sure my steam update was also ridiculously large, as if it redownloaded the whole game, which is actually what happened I think.
I checked the hard drive afterwards, and it only got heavier by about 30 GB, but that was after the patch was installed, and I distinctly remember the update saying it was like 80 GB.
So I don't think this was GOG's fault.