Yeah, I really don't have a problem this. It really isn't any worse than the little people in American wrestling. I don't follow that stuff much but I remember seeing an Irish duo who occasionally gets a leprechaun to come out and help him fight. Frankly, I don't see too much of a difference between those two things to criticize the Australian event while not criticizing the American one. The thing at the races was, in my opinion, no worse than giving a your child a piggyback ride. If someone is short enough to ride you like a horse, or at least pretend to, why not? It's more discriminatory to say 'no you can't do that because you're a short person' than to let them get on you're back.
I'm finding it a little bit difficult to see how this causes offense though. If anything, it's holding up little people as a better, maybe even perfect, group of people. People ride horses and most would argue that people are better than horses. Little people ride standard height people so it's kind of possible to make that link. Maybe that's what causes the offense here, standard height people feeling afraid that little people consider themselves equal to or better than themselves. Maybe, but probably not.
