I would like to point out also, that many people who would be described as "skinny" would almost never become obese. Neither would they be able to put on much muscle mass. It is a huge misconception that our bodies are just blank templates that can become fatter or more muscular. Different bodies have affinities for different shapes.

Someone who is very ectomorphic and "scrawny", will often have severe difficulty gaining a substantial amount of weight.