From your Outlook, go to File / Account Settings / Account Settings
Another window will appears and from there you can press Change/More Settings/Advanced/Outlook Data File Settings/Compact Now
It will be necessary to wait for the loading to finish completely (It can takes 3 to 4 minutes)
Once the compacting is over, you can restart your Outlook, and do the procedure one more time (Compacting should be a lot faster)