How to compact the Outlook PST file ?
When you delete email in Outlook, and empty the Deleted Items folder, the messages still remain in the Outlook.pst file. While Outlook periodically reclaims that wasted space in the background, you can manually initiate the process. This makes good sense after deleting many messages, or after moving year's worth of mail to an archive PST file, for example.
To manually start reclaiming wasted space in an Outlook PST file:
1. From the Outlook menus, select File > Data File Management
2. Highlight the desired PST file, click Settings > Compact Now