Why do deleted Messages not clear iCloud storage?

My devices (iPhone, iPad, and Mac) are synced for text messages, and I back up to iCloud. I have approximately 30 texting strings saved, occupying less than 1 gig of memory on the phone, including attachments. Yet my iCloud storage account shows more than 19 thousand (!!) messages saved, using more than 11 gigs of memory. Does deleting a text on my devices not also delete that text from iCloud? Why wouldn't it? And if not, how do I clear that space in iCloud?

For context, I do have Keep Messages in iCloud set to "Forever", as there are some I do want to keep, meaning I don't want messages that I have kept on my devices to automatically disappear. But surely this doesn't apply to deleted messages. Right?

Any insight would be appreciated. Thanks --