How can I clear iMessages storage on my iCloud?

Yesterday, I got a MacBook, and when I logged into my iCloud, all my old messages showed up. Then, on my iPhone, I noticed I only have one active chat in the IMessages app, which takes up 12GB. But in the settings, it says the IMessages app is using 19GB of my iCloud. Also, I’ve already deleted the conversations from the trash and my back up is update

[Re-Titled by Moderator]