I'd like to clarify some of this, since there I see a lot of misconceptions online about the points that have been raised.Size truly doesn't matter as long as you're at your body's healthy set point. I do believe that most everyone has a set point weight and it can be achieved and maintained through healthy eating and exercising. Some people can be rail thin, eat like a horse, and be healthy.
On slim people "eating like a horse": This is only true of teenagers, especially boys. People who stay slim easily often perceive that they eat a lot of food, when in fact they're eating a healthy amount for their height, body composition, and activity level. On the other hand, people who tend to be overweight often perceive that they're not eating much food, when in fact they're consuming more calories than they expend. These differences in perception have to do, in part, with the ability to differentiate feeling stuffed from feeling full. A lot of us are taught that they're the same thing, when in fact feeling stuffed is usually a sign of having over eaten (unless you've been gorging yourself on celery).
As for set points...Someone who's 5'6 would struggle to reach and maintain a weight of 100 pounds. However, no one's set point leaves them at a statistically unhealthy weight and body composition. (It's quite possible to be obese and have healthy cholesterol levels etc., but it's not common.)