Can someone in the know explain to me why the Steam download is 10GB, and 17GB on GOG? It's not a huge game, thankfully, but why can't GOG offer the same compression of files? Is it a dev or store issue? Any insight is appreciated.

Can't help you there, I'm afraid.
However, thanks for pointing out the difference. Could mean that GoG actually got a different build (the one with the reinstated uncensored cutscenes?) after all.
7GB is a bit much of a difference just for that though.

No, I really think that Steam files are one compressed folder and that the BINs on GOG are uncompressed.
I still prefer to have the installer, but it can be annoying for huge games.

Terminator Resistance install size via Steam is 28.15GB. What are you even talking about? https://steamdb.info/app/954740/depots/