After deleting a large number of messages, you may notice the size of your pst doesn't decrease. This is because it needs compacted. Normally, Outlook does this when the computer is idle and the space used by the deleted items (aka "white space") is over 20%.

You can initiate a compact at any time by right clicking on the Outlook Today icon in your folder list or on the Outlook bar and choosing Properties, then Advanced and Compact Now.

If you have multiple personal message folders, right click on their top level folder, where you account name is displayed and choose Properties, then Advanced to compact them.