Game updates fail due to low disk space even though the game's SSD has plenty of free space (over 72GB).

The download tab in GOG Galaxy app shows a generic "Update failed" message with no mention of disk space at all.

The most recent example for me has been Cyberpunk 2077 (trying to update to patch 2.21).

Does anyone know if GOG Galaxy uses the original full game's install size as a space requirement (84.6 GB)?

I have not seen such behaviour in Galaxy yet, BUT I know from sad experience that the offline installers use the windows temp folder to unpack. I would not be surprised if that were the case for downloads for updates as well, especially since the update files probably are supplied by the publisher and not GOG. Plus, you likely have your Galaxy installed on another drive than the games themselves, so it might try to unpack in a sub-folder there. And with CP2077 being relatively large, there might have been space issues creeping up on that drive, and the update might still be a large file if they updated a bunch of textures.

Another possibility is permissions, maybe the game was originally installed by another windows user account and now you have read, but not write access to the game files. And given todays lack of proper error detection and -reporting, that may also end up being declared as "space" issue.