It appears that some attachments get stuck and aren't offloaded like they are on iOS. I have 200-300 attachments, all dated 5/3/2021 (perhaps that's when I last formatted this Mac and downloaded from iCloud), and then it keeps 1-10 attachments from every month after that. I don't want to delete the attachments because I want them to stay in iCloud. Is there a terminal command to offload or set the maximum space Messages may use or something? Signing out and signing back in does nothing. They stay on the hard drive.
Messages on my phone with the same synced conversations stay around 21GB.