IF the submissive WANTS to be exercised and trained and wants to be reshaped, sure, it can be a great relationship. But if the submissive only thinks s/he wants that or thinks it's worth it to gain a great master, there will be nothing but emotional pain and disappointment for both.

Secondly, even if desired, the journey is going to be hard on the relationship as anyone who is overweight, especially the chronically overweight, will tell you, failures are common and self recrimination intense. The most likely result is to end up with a broken subbie and a broken relationship. I'd rather have a happy subbie than a miserable subbie trying to get thinner.

And even if all goes well, the relationship will always be strained because of the actions and humiliations perpetrated by the demanding dominant on even a willing submissive. Pain and humiliation work in bdsm because of the sexual aspects... which won't happen in this context.

My opinion and even if I were into micro-managing my subbie's life, (which I'm not,) I still wouldn't enter into a body-morphing relationship... not without being paid like a professional trainer.