A data file is a Personal Folders file (.pst). If you delete items from a data file, the size of the file will not change unless you compact the file. Compacting the file might take several minutes.
On the File menu, click Data File Management.
Click the data file that you want to compact, and then click Settings.
Click Compact Now.
Note: When you compact a .pst file, you do not have to exit Microsoft Outlook.