reducing Messages in iCloud storage

I am trying to reduce the amount of iCloud storage consumed by Messages.

My wife and I are both using Messages in iCloud on our iPhones. We also both use iCloud backup on our iPhones. From what I can tell, if you use Messages in iCloud, then messages are not included in your iCloud backup because they are already stored in iCloud. Similarly, if you don't use Messages in iCloud, then your messages would be included in your iCloud backup.

I'm trying to reduce her iCloud usage for Messages. She has gone through her largest threads and deleted photos/videos but it's not reducing her iCloud usage for Messages. I figured deleting photos and videos from the Messages threads would reduce storage but I'm not seeing that in practice.

Additionally, we are both on a group Messages thread with two other people. The thread consumes 949 MB on her iCloud but only 500 MB on my iCloud. It's the same exact thread on both devices, why would the storage consumed be different?

Any advice for reducing iCloud storage used by Messages? We keep messages forever, suggestions to reduce to "past year" aren't helpful to us.