Back when I had Thunderbird in Windows XP, I used to compact mail folders every now and then (click "File -> Compact Folders"). Quote from a Thunderbird web site: "When you delete or move e-mails from a folder, Thunderbird and other e-mail programs simply hide them (and mark them as ready for physical deletion later on), and these hidden e-mails still actually remain in the folder. Even emptying the Trash does not physically delete them. These hidden e-mails are physically removed only when the folder is compacted. If you don't compact your mail folders periodically then they can grow very large and erratic program behavior may occur." Is this an issue with the OS X mail application? I can't find any mention of compacting folders anywhere in the application.