If you turn on message tracking you will be able to determine who is receiving mail, and what distribution lists are receiving the most email. Tracking logs can be used to determine the highest volume of senders, as well as the largest recipient. You will have to filter the data yourself. Distribution lists show up just like normal recipients. The best way to determine this is to make sure your distribution lists all have a similiar email handle. For instance append all distribution lists with "dl-". Then you could very easily use message tracking logs to find the highest receiving distribution list.

MS also has a tool called MTLanalyzer.exe that will run some basic canned reports on the tracking logs. You may have to contact MS support to get a copy of it. We got it from our alliance engineer.