1031D, logically (just trying to imagine the physics) won't that also make the tires have a "rounder" tread surface? Hence the diametrical (OD) increase to make the tractor stand a little taller. If you put a piece of paper on the table and with one hand on each side push the edges of the paper toward center the center of the paper rises up off the table. Same principle here right, or amd I thinking about this wrong?
Also, you will probably know, are the 18.4 tires what they put on the rice models? I remember it had (to a little kid) very big tires.