One thing I found helpful is Markins' rating of both weight and torque, and the combination of these two numbers telegraphs their measurement/rating system. Let us accept Markin's own spec sheet that both of the following to be true at the same time (this is for the Q20):
max torque= 220 inch pounds
now Then the measurement has to be made by hanging 110 lbs at 2-inches away from ball center. I imagine this is the absolute maximum torque that the ball can tolerate without either slipping or becoming damaged. not realistic at all, but it does give you a clue as to how to determine the real capability. Hats off to Markins for at least revealing this.
now then, the distance from the clamp itself to the ball center is approximately 2", but mount a real lens on there and you quickly see that 110 lb capacity disappear. for example, my 70-200 f/2.8 produces 5" of distance from lens center to ball center. This configuration would only support 44 lbs of camera+lens. 44 lbs times 5 inches = 220 pound inches.
so put a larger lens on there and measure lens center to ball center. divide 220 inch-pounds by this distance and you will have the absolute maximum weight of gear that the ball will tolerate.
Note that torque is computed using the distance perpendicular to the the direction of the force. When the camera gear is directly above the ball, the additional distance the lens is above the ball doesn't doesn't add to the torque - it's the horizontal distance between center of ball and center of gravity that generates torque.
When shooting macro and aiming on the ground - or shooting sky objects and aiming the gear straight up - your 5" will be the distance that matters.
So in the end, the type of photography done will affect how powerful ball head that is needed. People only shooting while aiming their gear horizontally don't need as strong/rigid head as if they shoot aiming straight down or straight up.
This is also a reason why gimbal heads can be very nice to use for heavy equipment, allowing it to swing up/down having the rotation axis much closer to the center of gravity. Easier for both the head and the user.