It's just a sad way to live (in addition to the jerky way they go about it), and a way to set oneself up for a whole life of disappointing serial monogamy as they continue to chase the shiny. Not wanting to start a debate about the value of exercise in general--it's just this idea that you can get into a long-term relationship and you can somehow micromanage the person into never changing. Even if they stay exactly the same weight forever and ever, they will age. They might develop health problems. They will change emotionally/mentally. And so will the person wanting to do the controlling. It's just part of being human.
Well, I think the thing is, to these people, a partner is an accessory. Their ego demands that they have the brightest, newest, thinnest model - kind of like a cell phone . The idea that their partner might become less physically perfect threatens their own self-image. They live in fear of being "that guy with the dog of a girlfriend". The girl isn't as important as their friends' view of them as "winners" in the competition for Hot Partner.
DH has a friend like this. He doesn't try to control his girlfriends' eating as far as I know (although he is always on a diet himself), but whenever he starts dating
someone he must
receive confirmation from his friends that the woman is "hot." He was very frustrated with DH when he started dating
his current GF and his repeated attempts to get DH to agree that she was hot were met with generalities along the lines of "she's definitely your type." He's since paid for her to get breast implants.
One comment of his that has really stuck with me was when another friend mentioned that his wife was going to have a C-section and SuperficialGuy blurted out, "I would never let my wife do that! She'd have a disgusting scar!"
Way to prioritize, buddy.