As your Outlook data file increases in size so does the risk of corruption and lost email, contacts and calendar. To see the size of the data file, go to the location of the file, (the default location is dependent on your operating system: Win 7: drive:\Users\<username>\Documents\Outlook Files; Win 8: drive:\Users\<username>\AppData\Local\Microsoft\Outlook), and move to the right to see the size of the pst (data) file. In this example, a 20 GB pst (data) file became corrupted. When the user went to restore the backup, the file was older than they had hoped. Before you go to the next step, please backup your pst (data) files
Very large files cause problems because of the way Outlook handles the data. To resolve this create a second data file and move older email folders and saved data folders to that new file. Set the second data file to not send or receive. In time Outlook will reduce the original data file, (you will not see an immediate reduction in file size as Outlook will compact the newly created open space over time). In this example the 20 GB data file was divided into two smaller data files. The primary one used to send and receive e-mail is no roughly 8 GB and the secondary data file, the one that holds all the saved e-mail is roughly 12 GB. If you feel the secondary data file is still to large, or will become very large in short order, create a third data file, and so on.
You should see the backup software handle the smaller data files better. Your Outlook performance should be faster, especially is areas like Indexing, (Searching).