You could probably project that to 95/5 in the case of emails, where the expected case is miniscule and the worst case is huge. If you manage to go through a gig of inbox space in a couple months, there have to be some massive files there accounting for the bulk of your storage.
I'm inclined to agree with this fraction. I just used POP3 to LIST the first 360 messages in my mailbox and the graph shows an distinct upshot around 95%.
Around 95% of my (360) emails consume 6.5% of the space (360 messages take).
