I'm no expert here but isn't it more likely the memory consumption is coming from the feature dictionary, and feature-label matrix? Because the label count indeed is fixed and small.
This is a great job for a profiler like JProfiler -- would let you easily see where the heap is being consumed. A bit more manual, but free, is the jmap tool in Java: http://www.startux.de/index.php/java/45-java-heap-dumpyvComment45 A quick moment with this might easily demonstrate what's taking the memory. Resetting the data store might well be the right thing to do, if my guess is right.
