a good friend of mine worked at a halter QH barn when she was younger, early 20s or so. she's very much ashamed of what she's done now, and regrets it, but here's just a little bit of what they do to get these horses to be beefy abominations...
the horse is to be blanketed with two or three very heavy duty blankets and made to lunge for approximately an hour. it's allowed to cool down for a bit and the process is repeatedly several times a day. mind you, this takes place no matter the temperature... so these horses are made to sweat in 95 degree weather for 6 hours a day wearing 10 lbs of canvas and felt on their backs.
in addition to that, mares are injected with testosterone to promote muscle development... but it also makes them nicker at other mares.
after a while they begin to behave a bit like studs.
I'm not knocking showing halter, I have no problem with it. I think any discipline has its good and its evil.
But those horses are way too 'roided up. They remind me of those world body builder competitions on TV... disgusting.