I have Outlook set to clear out my Deleted Items every 14-17 days. I got so much mail over the past couple weeks that Outlook reached the maximum allowed size for an Outlook 2002-formatted file. Almost every operation I tried, including permanently deleting mail from my Deleted Items folder, resulted in the error "the folder is too large." The only thing that worked was copying messages to a different file, so I had to create a new Outlook 2003-2007 mail data file and copy everything over.
I guess it's a good thing they fixed their file format in the previous version. Otherwise the spammers would have won.