Frame rail spacing question

Does anyone know why the distance between the frame rails of a K/KB 1,2 and 3 is roughly 10 inches greater than that of the larger models. Does it allow the cab to sit lower on the frame, perhaps?

maybe to allow for the dual wheels on the rear?

I knew if I waited that someone would come up with a plausible answer. That has to be it. Thank you.

Frame rails on larger commercial trucks are a standard width (34"? I can't remember off the top of my head) to allow any standard truck bed or body to be mounted. Smaller Pick-up frames are wider and usually not a standard size/width. And yes, narrower to allow for dual wheels and suspension.