You can’t look at anyone, fat, thin, and in between, and assume what they eat or don’t eat, or what they do or don’t do for activity exercises. There are so many factors that go into changes in body weight.
But it’s so important to remember the difference between causation and correlation. And it’s also incredibly important to remember that fat is rarely a cause of health problems, but very, very often (though not always) a symptom. Unhealthy eating and lack of activity can cause people to gain weight. But that’s the important word—it can. As in, not always. Lots of people are technically ‘unhealthy’ and at a ‘normal’ weight. Remember the girl who collapsed after eating nothing but chicken nuggets?
You can’t put a weight on health. You just can’t. Thin people can be unhealthy. Fat people can be healthy. And there’s nothing subjective about it, it’s fact.