This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I downloaded a trial version of Jprofile once to something like this. I don't remember specifically looking at how much memory was going to the session objects but I think this tool would allow you to see it.