Outlook 2011: where are mail message in my archive file stored?
Outlook 2011 for MAC stores only the indices in a database file and stores the data records (individual emails) as a series of files nested within folders. The Database file and its folders are located here:

~/Documents/Microsoft User Data/Office 2011 Identities/Main Identity/Database
~/Documents/Microsoft User Data/Office 2011 Identities/Main Identity/Data Records

Adding these path's to your backup strategy should address your issue.