Gmail storage on iPhone unchanged even after deleting ALL the emails

G mail consumes nearly 18 gigs of storage on iPhone 2020 SE. OK, so I delete ALL emails (save for a dozen plain text messages). Yes, the trash, spam and sent folders are deleted and empty. After emptying everything, there is just a 500 MB reduction in storage space. In fact, the amount of space is now GOING UP after deleting all messages. Folks, I don't want to delete and reinstall anything - I just want to access that 18 GB and delete most of it.

What's up with this? How can this be resolved. Action steps please.