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. In reality, from over 3300 cases of workplace bullying reported to the UK National Workplace Bullying Advice Line, the incidence of overweight people being bullied is no more than I'd expect to find from a random sample from society.
Females are especially prone to comments about their weight.
Society's expectation and aspiration is the slenderness of Kate Moss and other supermodels, although this starved look is probably unhealthy and generates suspicions of anorexia or bulimia. What has struck me is that often the female serial bully is fat, and chooses a slim female target on to whom to project her self-loathing. Envy is a strong motivator for bullies. Even if you start a weight loss program, the bully will simply change the excuse. Each reason is specious.