There is a rough equivalent in the world of horse flesh to the insecure woman who constantly inquires whether a certain garment creates the perception of a large posterior. This is the phenomenon of the bigger guy who is convinced that only a horse of immense proportion can accommodate his frame. He will worry anyone who will listen that a 1,200 lb, 16 hand tall horse will immediately buckle under his 225 pounds. That only a Clydesdale or something approaching that size could possibly put one hoof in front of the other while he is in the saddle.
They almost always couch this in terms of concern for the poor frail animal but personally I think they are having a little trouble with an over active fantasy life or that they are just afraid of looking like this guy.