I've added a link to the Memory Usage/MessageSoftening wiki page to the FAQ,
and also added some big bold letters at the top of that page just to avoid
any possible misunderstanding of what it does.
How much memory should a soft reference use?
Using the default cache settings, my memory usage (based on the Windows Task Manager) was about 200MB more when there was 100,000 messages waiting to be consumed than when the queue was empty. The total java memory was at 300MB. So, assuming that most of the refrerences were soft, this would indicate that each soft reference was using 2K. Does this sound right?
I didn't look at the actual MBean properties when the JMS queue was in that state, but I plan to build up the queue to 100,000 messages and try it again.