Even though you delete your e-mail from the Delete Items folder, the file size may still stay the same. This means that you need to manually compact them.
1. You need to have the Folder List showing. If it is not already there, from the View menu, click Folder List
2. From the Folders list, right-click on Personal Folders at the top
3. Select Properties For Personal Folders
4. Click on the Advanced button
5. Click on the Compact Now button