It's PM BOX Chaos!
Boxes seem to be filling up rather quickly. I think I found a possible problem, by default when you send a PM you'll notice some checkboxes near the bottom, one of them says "keep a copy in sent items folder".
Most people when they go to delete their old PMs delete the replys, however, all those other PMs are still in the "Sent Items" folder.. Clear them out people, I didn't know that until today. Hopefully this will resolve my problem with full boxes as well.