I've always wondered why some people put on weight easier/quicker than others? what's the science behind that?
Another reason can be hormones, for example Thyroxine (people who suffer from hyperthyrodism, over active thyroid gland, will tend to be thin, fidgety, rarely feel the cold, etc; those with hypothyroidism, underactive thyroid gland, will tend to be fatter, tired, lethargic, feel cold).