So I cleaned up that 15 GB mailbox a bit. I had previously deleted all messages over 500KB in size. Still several millions of messages left (all from 2014 to 2018.) Don't even know how to count them, because Gmail counts only conversations. Maybe it can be done via IMAP or the API, but I'd expect hitting limits on the way, so I did not spend any time trying.
The irony is that Gmail cannot delete more than a couple of thousand messages from Trash/The Bin in one go. It says "All messages deleted", but when you refresh you see that it's not true. You need iterations. I guess they are using something what AWS calls a lambda to empty the bin (don't recall the Google term) and that job hits a limit.
Yeah, I don't use that mailbox for anything serious at the moment. Just an experiment...