When I fill a Choice with more than 30000 Strings, why doesn't it show all of them?

The AWT Choice component is limited to the capabilities of the associated native control of the user's platform. Sounds like it has a 32K limit for your platform.

As a user, I would hate the user interface if I was expected to pick something from over 30,000 choices. You might consider designing an interface that limited the number of options to something a little more reasonable.