Messages filled up my MacBook Air

Hi all,

About 6 months ago I bought an M1 MacBook Air. It's low end spec - 256GB SSD, but has been running awesomely since my purchase. Today I was doing a long sync with a 3rd party app called Garage Sale. After several hours [I have a very slow internet connection], the sync failed and I noticed I had just a few hundred MB in space left. I assumed it was Garage Sale, but nope - it is Messages! I have Messages in iCloud enabled on my iPads, iPhone, this MacBook Air, and my main Mac Studio. I've not had Messages run amok like this on the other devices.

I have signed out of iCloud on the MacBook Air in hopes that that might flush the messages on my Mac out. I do have "keep forever" enabled on Messages in iCloud but since my other macOS and iOS devices have not been using 151GB in Messages, I am unsure why this suddenly happened on the laptop. It is running macOS 14.6.1 [one of my weekend projects was to install macOS 15 on the laptop, but I have to sort this issue out first].

Since logging out of iCloud, the laptop is slowly reclaiming disc space, but the Messages still show up using 151.24GB.

Looking at my Mac Studio which has 2TB, I notice it too is taking up a lot of space - 167.81GB. There are more attachments downloaded to the Studio than the laptop, but I hardly use the Messages app on my laptop - I don't need access to a large attachment from 2 years ago on a daily basis.

I want to preserve my message history - eg: I do want to keep my messages - but I do not want all of them to fill up over 50% of my SSD! It's a 256GB SSD and I was using just under 100GB before I started the Garage Sale sync.

Is there anything I can do to free up the space taken by Messages while signed out of iCloud on this Mac? I now have 13.77GB free, but still 151.24GB in Messages. Updating to macOS 14.7 requires 19GB of free space. The free space on my SSD is slowly going up - since typing that last sentence it is 14.01GB, but the Messages space has not gone down at all. Help?

Thanks in advance!