I figure I'll toss my 3 cents in here. (Inflation don't you know.) The numbers on the side of pickups are the maximum load they can handle. Now, you can option them to haul more weight or pull better, but the standard truck is what the numbers on the door say. A 1500 can handle 1500 pounds no problem. The brakes, chassis and drivetrain ad designed for it. I have a Ram 2500 and a F-250 both. The Ram will carry much more weight with no problem than the Ford as it has the heavy duty chassis package. It also rides like a log wagon. The Ford is an oil burner and will pull with the Dodge and pass it when the Dodge stops for fuel. It also rides better and has a sharper turning radius. Both trucks have a minimum load of 2500 pounds maximum. The Dodge can carry much more due to the optional chassis. It basically a 1 ton truck with single rear wheels. It has triple sets of springs in back compared to a coworkers Cummins Dodge or my F-250. I drive the Ford more because it rides better and gets better mileage.
The chassis on both trucks is much heavier and more stout than a comparable 1/2 ton. As another poster mentioned, the rear axles are completely different between 1/2 tons and heavier trucks. GM is bad about putting a 2500 rating on semi floating axles. The semi floating axle carries the weight of the truck and load. The full floating axle (that big tube that sticks through the wheel on heavier trucks) doesn't carry any weight, just turns the wheel. The rear wheel bearing carries the full weight of the truck and payload.