Pigeon shooters, like field/game shooters, come in all sizes. The concept of larger, heavier, more powerful guns must be relative to the shooter's size. A person of slight build is not going to muscle a 7 3/4# gun as well as a 6 1/2# gun. For the person used to a 5 1/2# game gun, a 6 1/2# gun is bigger, heavier, and can be more powerful. So, the decision is to accept a lighter charge from a smaller gauge to get a gun with lower weight and less swing effort.

Logical, but I don't know if it is the total explaination.