Hangouts is using obscene amounts of background data 248 Mb this last week of background data compared to 41 Mb foreground. Last month was 400/100. How do I go about seeing what's the cause? I don't send/receive many pictures, when I do it's usually a Hangouts message, not MMS, but usually on wifi too. My brother is 441 foreground, 85 background fore comparison, so about opposite. But as frequently as we're on wifi, I wouldn't think it should use that much cellular data.

Anyone have any tips or a way to start investigating what's going on?

Edit: doesn't fix the root cause, but disabling background cellular data usage doesn't stop you from getting notifications through the magic of GCM push notifications.
It used 1.5GB in less than a day for me a couple of weeks ago, so I've turned off background data for the app. Makes it pretty crippled, but only option. I can't find any answer to this problem anywhere.