Hi SASanderson,
Per the article Delete messages and conversations in Messages on Mac:
"When you delete a message or conversation, it’s moved to the Recently Deleted folder. You can view and recover items in the Recently Deleted folder for up to 30 days. After 30 days, the messages are removed from all your devices. After 40 days, the messages are permanently deleted from iCloud.
- In the Messages app on your Mac, choose View > Recently Deleted.
- Select the conversation in the sidebar.
- Do one of the following:
- Recover a recently deleted message or conversation: Click Recover.
- Permanently delete a recently deleted message or conversation: Click Delete."
Messages that are deleted but still retained in the recently deleted folder would continue to utilize storage. You may try the process from that page to permanently delete these items to recover the space, and if you have further concerns or questions contact Apple via Contact - Official Apple Support.
Regards.