I get occasional messages from Gmail informing me that I need to manage my mail or start paying for additional storage.
Some of my messages are hogs for storage. I get video files from friends and I send photos from my phone to my laptop via email.
If you want to identify the emails that are gobbling up your space, go to the search line at the top of the page and insert the command "larger_than:NUMBERmb" where you supply NUMBER. (Info here)
In this case I chose 15mb. I wanted to shave my way down from the top and look at the kinds of emails I was deleting. I decided I wanted to get rid of all of those emails so I mass-selected and moved them to trash.
Then I searched for emails larger than 12mb, then 10mb....then 5mb. All sent to the old byte-bucket in the sky.