First of all I would like to suggest you to archive your PST file, so that you can reduce the PST size upto the Outlook version limit you are using. Because backing up Outlook will create an exact copy of the previous file but not reduce the size of your file. Follow the steps to manually archive PST file.
1. Open Outlook>>File>>Archive option
Or
Go to Tools>>Mailbox cleanup>>Auto archive
from here you can archive PST file according to the date, time, month.