I'm pretty sure that when they came out with the 85 sq. inch graphite frames a couple of decades back, they actually called that hoop size a mid plus for a while until the bigger ones came along. Prince had the oversized frames early on in the post-wood movement of racquet development, too. I guess that there was enough of a difference among all those metal and graphite models that they had to come up with three different general terms. I wasn't in the room when the powers that be made that decision, but that's the way I remember the terminology progressing.
As I understand it, the mids are anything from 93" on down, a mid-plus is usually in the 95"-100" range, and bigger heads are called oversized. I suppose there will be some debate if anyone produces a 94" frame, right? Just to throw a wrench into things, Prince had their O3 Tour models and referred to the smallest of the three versions as the mid, but it was a 95" head.
Oh - I was writing while VGP nailed it. Thanks dude.