What else is running? Are you syncing to icloud or anything like that?

Unless you brought down that amount of data files, I very seriously doubt it was from just polling the exchange server.

You could also try a network monitor like little snitch at

http://www.obdev.at/products/littlesnitch/index.html

which will tell you what is using how much of your internet. The free trail is only good for 30 days, and only for monitoring three hours at a time, but that may help.

Depending on the Mifi you may have the ability to see packets or data coming in and out, I would suggest shutting down everything and bring your software up one by one and see if you can identify what is causing the jumps in data usage if you can't track it a network monitor.