Monitoring and limiting internet usage
Solution 1:
If you install Little Snitch you can monitor which application using internet. Every time any application wants to use internet you must accept or deny it, but it doesn't have an ability to measure bandwidth usage of each application.
You can see its demo here.
Solution 2:
600 MB doesn't sound like iTunes, unless you told it to do it.
I'd look into "Software Update...", which, in its case, can have its automated run turned off so you can run it manually.