How do I delete large attachments from iCloud messages? Deleting them locally does nothing.

I have 4 GB of large Messages attachments taking up space in my iCloud Drive. Apple's instructions on managing iCloud Drive storage space seem to be written by someone who did not understand the distinction between local and cloud storage, so if you have your iPhone configured to only store recent and relevant conversations, there is no way to manage or access or delete old attachments. Can someone explain to me how to delete old attachments if my phone is only showing 50 megs of usage and my iCloud is showing 4 gigabytes?