I saw a question on the mailing list asking for a simple, stripped-down, version of ActiveMQ. The default version 5.7.0 distribution and all of its runtime dependencies come in at about 44MB. I was able to get a working, "lean" version going at about 7.4MB.
First of course, some of the requirements:
Here's what my $ACTIVEMQ_BASE/lib folder looks like
Give it a shot! If someone spots more dependencies that can safely be removed, let me know!
UPDATED: As Gary pointed out, all the spring deps can be eliminated if doing embedded broker. I've put together an example here: at my github repo