Why is the All Mail folder more than twice as large as the sum of all other folders?
The Gmail All Mail folder includes messages from all the other folders except for Junk and Trash. It also contains messages that don’t have any Gmail labels like messages that have been archived.
If you login to your Gmail account using the webmail interface and go to the All Mail folder. Look for messages that don’t have a label and they will add up to the size difference.