If everybody filled their mailbox, 500 PB would back up 66.1 million Gmail accounts. Since they don't, we're probably looking at everybody on Earth: 73.8 MB per mailbox, which sounds like a pretty good shot at an average.
Looking at the SL8500, I think I'd intentionally lose data just so I could play with it to restore it.
There's probably a lot of people that don't "get" Gmail and delete all of their messages. There's probably also a lot of people who sign up for an account and don't use it, ever, and they contribute to the numbers. Keep in mind ~80 MB is for everyone on the planet (all nearly 7 billion) in 500 PB.
I'm betting the average is a lot lower than we'd think.
74MB seems little to me too (I am thinking of how often people have sent me ppt of cute cats, word documents of a todo list and tiff scans of some thing they drew).
It would be nice if grandparent has some reference for such a number to share
Looking at the SL8500, I think I'd intentionally lose data just so I could play with it to restore it.