Although I do not support any type of genetic disease, bulk does not always equate to an inability to perform. I've seen some over bulked horses do quite well. I do, however, prefer my horses to be more delicate.
The question that comes to my mind is- Is this truly different? These horses are bred for a certain purpose. Assuming no health problems are involved, is this any different from breeding a warmblood designed for dressage? Take Quarterback, for example. Look at his conformation and gaits- few horses I have seen with these gaits. Why? Genetics, precise breeding, and evolving a breed to 'perfection.' These gaits are not 'normal' and typical, but more so man made. These halter horses have been designed to have significant muscle mass for a different purpose, but is it really fair to descriminate only against the halter horse as opposed to others?
Like I said, assuming they are in good health. Just something to think about.