How do people let their pets become morbidly obese?

Surely they are their only source of food, so just give the dog the right amount? And if you somehow went wrong, surely you can easily put them on a diet and very quickly get them back to normal weight?

I have seen one of those dogs (small type with squashed face) that was incredibly obese. The breathing sounds it made were horrible. I felt so bad for it.

Just watching the effort it’s putting in to walk!

It seems so cruel. Yet in the specific example I am thinking of, I know they love that dog, they don’t hit it or anything. But they presumably don’t realise that they are abusing it in another way? I almost would say it’s not any better than hitting a dog, because it’s going to reduce its quality and length of life.