Those who can, do. Those who can't, bully. People get bullied and harassed for all sorts of reasons. A commonly reported reason is because you are, allegedly, overweight or too fat.
Society has a prejudice against fat people.
However, if you start a weight loss program and lose weight, it may make no difference. As children reach teenage years, they are particularly sensitive to the criticisms of others. In 1998, Kelloggs ran an advertisement suggesting that if a child was being bullied because they were fat, then losing weight by eating cornflakes might cure the problem. Weight features prominently in cases of bullying. Some adults are bullied because they are perceived, by the bully, as overweight.