This is all my fault. I was running a bit short (to the point were two week old PM's had to be deleted), so I requested some more space. Apparently I didn't just get the extra space, but everyone else did so too.
The limit is 300 for each folder.
I'm not complaining.
I have a handful of PMs I am keeping for prosperity, and now that takes up a much smaller section of my inbox, which I only get around to cleaning out once I reach 98% or so.
Plus I've had important conversations that end up being 30+ replies, which takes up a lot of space, or used to. After the last layout make over the 'Message history' feature was added, which means you only have to save the most recent PM and you can still read all of the conversation prior to that message as well.