I have a similar problem. Last month, on one particular day, 80GB was downloaded (and another day, almost 40GB). This was definitely not something I was downloading - I don't even have 40GB of spare disk on any computer at home!
After resetting router passwords (in case someone was hacking my wifi), and monitoring all my home computers, I also narrowed it down to gmail, but only on my wife's and daughters accounts. One particular email from Pintrest caused 1GB to download in a few minutes before I switched tabs and it stopped. I reopened the email, but it didn't happen a second time. Also forwarded the email to my account but the problem didn't reoccur, so I'm not sure it was related to that particular email.
It's happened about 3 times in the last month, every time in Safari (I think, but I'm not 100% sure).
Has anyone else seen this, or does anyone have any ideas as to its cause?
(All my home computers are Macs, with the latest updates applied.)