The "fatness = bad/unhealthy" belief is so common days, and mostly unchallenged, that it appears self-evident. It appears as truth. But it is far from it. I spent 8 years reading about and analyzing "obesity" research during my PhD, and began to see that there is another side of the story that is quite compelling.
Read some articles that challenge these beliefs and make up your own mind. Also, at the bottom are recommendations for books that closely examining the research so that you can more accurately understand the impact, or lack of it, of fatness on our health and well-being.