I just got back from a few hours in the public library and enjoyed reading both of Knutsen's books. I think you'll find the scholarship in the 60s book is a bit off on some things and in some ways, but any area of research needs to start somewhere. In terms of answering your question on yari differences, Knutsen does suggest that certain yari cross sections and shapes were preferred by horsemen over others.
In addition, Knutsen is in line with what the members above are stating: that longer yari were for samurai who could commission them, or were given them as gifts.
I'll attach some pictures I took today.
I'm also speculating here, but if you are charging someone down with an Omi Yari, you may run the risk of over penetration, and then you either get yanked off your horse with it, or it snaps. A small stout one isn't going to result in catastrophes like that. A short one will obviously cost less, too.