PM boxes are being really abused and people just dont take the time or effort to clean out their boxes which ends up slowing the server down for everyone.
The best and quickest way to open up space in your PM box is to delete all the messages from your 'Sent Items' folder.
If people take the time and effort to keep their PM boxes clean, then we can bring the PM size limit back up again.
Again, we reduced the size of the PM box in order to keep the site speedy and responsive for everyone. So take the 5 minutes (really less than that) and keep your PM boxes clean