Our vet always gave us a very hard time when we came over with my grandmother's cat..
He also assumed it was bad care, not an ailment.
Although that might cause SOME cats to get fat, most of them are just being fattened by their owners.
And some people here did state that they have morbidly obese cats.
Ours got hit by a car when she was a stray, so her hip is severely damaged. She can run and play a bit, but not very well.
I still managed to get her down to 4 kilos. (She was 8 kilos when my grandmother died.)
I think giving them bad quality food is just as bad as giving them to much.
I tried to feed her whole animals, but she was too spoiled to eat those, so I switched to the best quality kibble I could find..
Cats aren't very expensive pets to keep. So buying them good food is the least you can do imo..
I'm not really much of a cat person though. So maybe that helps. Them begging for more and bad food doesn't have much of an effect on me. xD