One of my fantasies is to be completely dominated and enslaved by a thin/athletic/muscular woman. I'd be strapped to a bed, denied any exercise, and would HAVE to obey her commands to eat or risk getting jolted by the shock collar I'd be wearing.
Day after day, month after month, I'd watch her working out--getting thinner and stronger as more and more food was being shoved into my mouth and I was getting weaker, flabbier, and fatter. Eventually I would give up any resistance and willingly submit to her every food and sex-related desire, the restraints and collar would come off, and I would be transformed into a helpless blob.
She'd humiliate me for being so fat but still buy vast quantities of food, and by that point I would have lost all self-control and couldn't stop eating and begging for more.