Hi Orion'sBelt~

This is only a WAG, but I always understood the phrase to mean to express an unbelievably large amount. If, as you say, horses never choke and they routinely consume large amounts of food, imagine the volume of food it would take to make one choke. It is that great amount which is expressed in "enough to choke a horse."

When I have the opportunity, I will check my library for additional info on the phrase.

BTW - my neighbor has horses of various types. The miniatures are smaller than my dog.