When you delete or move messages around in PST files, the PST files can get fragmented. Outlook does compact the PST files slowly in the background but you should compact at least once a month depending on how many messages you delete. This will recover disk space back quicker as well.
To do this manually, right-click on the PST file you want to compact. The properties dialog appears.
Click on the advanced button and the following dialog appears.
Click on the Compact Now button and the progress window appears.
Depending on how large the PST file and the number of messages you have deleted, it will take a while. Once finished, click the Okay button twice to dismiss the dialogs.