At this point, hprof dumper iterates the fields trying the deduce the instance size. However, this is prone to errors in estimations, because:
 a) object header sizes can be changed without notice;
 b) object fields can be aligned/padded for correctness/performance reasons

We have already identified that with upcoming @Contended, hprof estimates can be terribly wrong, see:
http://cs.oswego.edu/pipermail/concurrency-interest/2012-November/010276.html

Luckily, VM already knows the exact instance size, and we can just reuse this info.