[PavlovsCat]

I think you can use a IMAP client like Thunderbird to clean it up.

(I say "think" because while I use a gmail account in my email client, I only use it to receive notifications from Google, so no wild folder/tags setup, and I never check the webmail to see if it matches the emails I see in Thunderbird)

[ahbyb]

Yes you can. You can go to the "All Mail" folder, add the "Size" column to the list view (right click the columns) and then proceed to sort every email by size.

https://support.mozilla.org/en-US/kb/message-list-columns