I have a very similar issue with iMessage in iCloud, and unlike photos, there’s no web interface. You have to interact with it on your device. Thankfully I have a Mac so I can load up the chat database from there and see what’s using the space, but cleaning it up has been a nightmare: I have a script to find the big attachments, but if I delete them then some agent that manages the database goes into a loop for like five minutes per deletion. Then the change gets synced to iCloud (at least, I assume). So unless I fully reverse what the deletion process does and whether it is possible to do a batch operation I’m basically at the mercy of the front end they provide to clean things up, which as I mentioned earlier is absolutely not designed to make it easy to do this.