Isn't Outlook (2003 and higher) not capable of compressing PST itself? Or does it throw away attachments?
Anywayz, There is an option that can trim it automatically in 2007 and higher (if memory serves me right). There is also a Free (and portable) version of Mailstore Home
with which you can create backup from your email. After having done so, you can clear out your PST yourself.
Best advice? When you're not connected to an Exchange server, stop using Outlook. Completely. Utterly. Immediately!
At least you don't have to work around this archaic limit anymore. If you do have to connect to Exchange, you're S.O.L.