Many thin people eat but have fast metabolisms and cannot put on any weight.
Many overweight folk are that way because they consume much more dirty calories then they burn.
Happy, jolly his is the stereotype, overweight people are "Jolly" and happy and skinny people are sad and miserable. Many overweight people that I know are self conscious and so build a loud happy persona to forget who they are.
Are you with them when they're alone on the couch gorging on ice cream, pizzas and pies or when they sadly see their reflection in the mirror and hate what they've became? Probabably not as jolly as you think they are.