Correct ,they work awesome on light pins because they have enough volume for the weight ,however on bigger and heavier pins there is not enough room for a bigger bag/ volume of air needed.
Result they are too hard when they are pumped up to support the weight.
Thats why air hitches will have at least two, EACH of which is bigger then the single one that fits a standard pin box. If one small bag is enough why do air hitches have 2 or 3? Same weight to support.
Henry's E.T. has a version with a huge single bag ,looks like it is 18 - 20 " across if not more ,I have never seen one in the wild.