If you delete items from an Outlook Data File (.pst), the file is automatically compacted in the background when you’re not using your computer if Outlook is running. You can also manually start the compact process immediately. It might take several minutes to compact an Outlook Data File (.pst). You cannot compact an Offline Outlook Data File (.ost). 

1. Delete any items that you do not want to keep and empty the Deleted Items folder.

2. Click File >  Account Settings >Account Settings.

3. On the Data Files tab, click the data file that you want to compact, and then click Settings.

4. Click the Advanced tab > Outlook Data File Settings.

5. In the Outlook Data File Settings dialog, click Compact Now and then click OK.