I just added the game to the GOG downloader, and the little popout on the "box" in the My Games section of my account said the game weighs in at about 593MB, and for a while that's how it showed up in the downloader. Now it reduced the number to 116.5MB. Did GoG slip in a different version of the game and not update the filesize or something? Is this a bug with the downloader? I'm using v0.9.30

Yep, it kinda had to be.
Anyway, good to know it's sorted now.