Looking to reduce jar sizes in Java5 and 6? Adam Bein has a very simple solution in his blog. His solution is to use the pack200 tool distributed with the JSE 5.0 and 6.0. Adam points out one good use for the tool.
The tool pack200 is especially interesting for the distribution of WebStart applications to achieve faster download times.
You can find a great example of this technology with GlassFish as it unpacks internal libraries using this utility.