To be fair, I would wager that fatpeoplehate had its share of trolls, but I would definitely believe that the vast majority of its denizens were former fat people, and very frustrated at the obesity epidemic in the United States.
Frustration leads to anger. Anger leads to hate. You know the rest.
I speak here from some experience. Several members of my family are morbidly obese, with BMIs well north of 45.
For the average person, obesity starts at a BMI of 30.
More than a third of the population of the United States qualifies as obese. Obesity-related diseases are the leading cause of death, and the problem is getting worse.
There are growing social movements that promote obesity as a way of life. This is simply insane. Nobody would promote anorexia as positive, or smoking, or heroin.
Now, this would all be completely fine if it only impacted those that choose this lifestyle, but the problem impacts all of us, in the form of higher medical costs, greater danger for service personnel, and in the cost of infrastructure changes required to support the obese.
Every holiday, I got to watch as some of the people I grew up with eat themselves into an early grave. To listen to them complain about their latest weight-related medical problem. To hear them make excuses and blame other people as to why they are sick.
And then these same people would insult my spouse and I, because we control our diets and exercise regularly.
I do not want to live in a society where this is accepted or encouraged.
While I don't think that mocking fat people will solve the problem, I can understand how that particular community could grow so fast.