Calendar size too big!

I have a couple of users who have Calendars that are around 50MB in size.  Our mailbox limit is set to 100MB, so that doesn't leave much room for other stuff.  I've looked around in their Calendars and can't find anything large, such as attachements, etc.  I've ran Mailbox Manager against each one and can't find much to get rid of either.  I even went as far as to set both the Age and Size of the Calendar folder in Mailbox Manager to 'Any'.  This query returns a report that says it can get rid of a whopping 309KB.  So where is the other 49.7MB of data?

I then went and tried something that may or may not add to this confusion.  I exported one of these large Calendars (44.7MB) to a .pst file and after the export the .pst was only 538KB!  I then deleted everything in the person's mailbox Calendar thinking I could just import the .pst back in.  Well, the Calendar size with NO items in it was 44.1MB.  So somehow the size is being reported wrong in Outlook?  Mailbox Manager seems to read it correctly (says it can only clean out 309KB).  The .pst file seems correct (only 538KB).  

Journaling is NOT enabled.  Something crazy is going on here - any clues?