Transit suspension

I own a 2005 3500 Sprinter w/ 200,000 miles. I am thinking about a Ford Transit 3500 medium roof. I am a electrical contractor & carry a lot of weight in the van & sometimes I tow my lifts around. I have looked at the 3 different Transits (1/2,3/4 & 1 ton) at a Ford dealer & all three seem to use the same tire & have the same leaf springs on them. Can someone tell me how Ford is getting a bigger payload on the vehicles?
Also has anyone purchased the dually version & how is the payload on this unit.

Thanks,
Matt