An Exchange 2010 mailbox database store or the public folder database store may grow unexpectedly large. This is due to whitespace not being reclaimed where objects are deleted or truncated during Online Defragmentation.
Microsoft has determined a possible issue when the following scenarios are implemented:
The previously available fix available for Microsoft has now been superceded by installing Microsoft Exchange 2010 Service Pack 1 and Update Rollup 6
Refer to Microsoft KB Article ID: 2621266 - "An Exchange 2010 database store grows unexpectedly large"