When you take PST backup in Outlook 2013 ensure your offline sync settings are set to ‘All Emails’ or you may wonder why your gigabytes of emails are making just a few megabytes of PST file!
Here is the reason in more details…
While using Outlook 2013 you might notice following message
There are more items in this folder on the server
Click here to view more on Microsoft Exchange
By default Outlook 2013 only synchronizes 12 months of emails to your Offline Outlook Data (.ost) file from the Exchange server. Rest email (older than 12 months) reside in your mailbox on the server
To change this default setting please follow this KB article