Actually, it's the height that plays a bigger role in terms of weight management. Generally, the taller you are, the more food you need. Hence, why most people who are considered to be rail-thin despite the fact that they appear to be eating so much tend to be 6'+ or just taller than the average person. In reality, it's possible for anyone to lose or gain just by eating the right amount of food for their height (no need for steroids!

), granted that you have no medical condition that directly affects your weight, such as hypothyroidism, Cushing's Syndrome, etc. As for other factors, genetics just usually determines where most of the fat/muscle is stored in your body, not necessarily the amount of it -- that one's all up to lifestyle, particularly eating habits and yes, exercise.
Just my two cents as a nerd/dietitian-in-training.

Does this mean I should be eating more cake and chocolate in the winter to keep out the cold?
And are just anorexic in general...