Moodle includes all the third-party libraries it needs, to make installation simple. Some of those are quite big.
New things in Moodle 2.x are the Zend web service libraries, and YUI3.
In one of the versions, I can't remember if it was 2.2 or 2.3, they went through and identified some libraries that are no longer used, and removed them, so hopefully the size gets smaller again. Hmm. Looking at http://download.moodle.org/ that does not seem to be the case.