What's the difference between regular and light 5th Wheel?

Old-Biscuit wrote:
Weight........
5th wheels are towed AND carried (pin weight)
For 5th wheel to be carried by a '1/2 ton' it has to be fairly light due to limited payload capacity of truck
Weight.......

Normally weight is associated with strength. In this case the lighter unit is warrantied to tow an additional trailer, where as the heavier unit is not. THAT I guess is my real question. Do they have the same frames, etc?