The foundation AQHA horse is not a tall horse. Very stout and very bull dog like in build.
I show year round in cattle events. Not many tall horses.
I have found the conditions/terrain dictate what style of horse is used on ranches. The short stocky old school quarter horses seemed very popular in east Texas where the places are smaller because they can run a lot more cattle on less ground. No need to trot 30 miles a day. Whereas on the desert where you need more ground to run a pair a leggier horse(but still big boned) is desired.
I agree that in the showpen a shorter horse is what you see.