This is a very good result. I experimented with your script and found
it so interesting that I spent some days for it. The challange are
obviously the obsolete classes - remanants of removed classes
that are still referenced somewhere in the image.
I have now a script that can reduce a  3.4 - #5170 image to
1.5 MB - without a single obsolete class. The reduced
image even keeps development tools, class organizations and
class comments, which is a requirement for use in schools.
When you remove these, too, you may be able to gain another 200 KB.
I placed the script on this page:
http://minnow.cc.gatech.edu/squeak/2182