F450/550 and Dodge 4500/5500 are considered light medium duty trucks, class 4/5, 15,000 to 20,000 GVWR. They are based on light pickup chassis.
GM C4500/5500/6500, Ford F650/750, F500/600/700/800 (gas), F5000/6000/7000/8000 (diesel), L700/800 (gas), L7000/8000/9000 (diesel), International 4600/4700/4800/4900, 4300/4400, Freightliner M2 series, various trucks by Sterling, and a host of other large chassis commercial trucks are considered medium/medium-heavy duty trucks, class 4/5/6/7, 15,000 to about 40,000 GVWR. These are based on heavy truck chassis.
Park an F450/550 or Dodge 4500/5500 next to any of the heavy chassis medium duty trucks and the difference in frame size, axle size, wheels and tires, transmission size is readily apparent. The pickup chassis trucks are dwarfed by these heavy chassis trucks.
The class 3/4/5 pickup chassis trucks have come a long way in their tow ratings in recent years, mostly due to increasingly higher output engines. Their chassis are still quite light and transmissions and axles quite small, compared to true commercial chassis trucks.
My F350 dually diesel 4x4 cab/chassis truck with 11' flatbed weighed 7500 lbs empty. My International 4800 diesel 4x4 cab/chassis truck with 12' flatbed weighs 16,000 lbs empty. That's over twice as heavy. That extra 8500 lbs is all steel and iron of the frame, axles, springs, transmission, engine, brakes, wheels, etc. The International is a massive beast compared to the F350.