Each one is designed to go on a certain gvwr chassis. # and size of outriggers is a critical factor as well. Every now and then some crazy customer comes along and puts one on a larger chassis than it was originally intended for, but usually because they don’t want to use outriggers and the larger chassis is needed for stability. They also fill up the blank spaces between the chassis rails with ballast in the form of steel plate. I always thought it was crazy to weld in 2000 lbs of steel lol. Most of the time they’d do it in the very back to also take some weight off of the front axle - seesaw-style.