I think the reason there is no size category longer than midlength is because even thirteen might not be enough to make a strong category, and also most stories over 200k are going to fit in one of the other categories. I think midlength is there as a category for stories that are supposed to be like a short book. Some of them would have trouble contending with a 500k story in the same category because the longer one has more time to really develop a story.
I've wondered about it too, But if you look at most of the stories over 200k that you would nominate, you're probably going to put them in another category as well, and also, between, for example, The Forgotten, Faustian Bargain, and It Might Have Been, since those are the only titles I can remember right now being that length, it would be extremely difficult to pick between them with nothing but size parameters.
