If you use
git clone --depth 1 -b MOODLE_21_STABLE git://github.com/moodle/moodle.git moodle
then that gives you a checkout that can later be used to pull updates, and that is 66.96 MB when zipped. Still double what you currently have.
However, there is a way to be clever. If you use
git clone --depth 1 -n -b MOODLE_21_STABLE git://github.com/moodle/moodle.git moodle
Then that creates about 42MB data, which compresses to 41.29 MB - since it is already compressed. However, if you do that, the moodle folder just not contain any files, just the hidden .git folder. You would need to arrange that when someone installs the Mac package, that it automatically runs the command
git checkout HEAD
inside the moodle folder. That will create the current check-out from the compressed representation hidden in the .git folder. That takes a few seconds.
Of course, that is only possible if the git executable is included in your installer, since I don't think you can rely on it being installed as part of MacOS, but I might be wrong.