It feels like past a certain point of fatness, you aren't actually into them for their apperance.
It's really just seems like a powertripping fantasy of being able to control someone's diet and how they look. The fact that they're so fat they can't move also makes them more dependent on you.
I get that it's just a fictional fetish but there are real life architects who unironically abuse their partners this way so it creeps me out.