One other way of finding how much memory is used is to enlist JMH's -prof gc option to see what gets generated between runs. I used this to see what StringBuffer did in http://alblue.bandlem.com/2016/04/jmh-stringbuffer-stringbui...

You can run the test with and without C2 (eg with -Xint) to see the effects on the inlining/escape analysis, and (on Linux) with -prof asm to see where the code is taking time.