It is Easy for Skinny People to Stay Thin
Theory: Skinny people have a higher surface area to mass ratio than the non skinny. That means that heat from their bodies dissipates more. Therefore they burn more calories relative to their mass in order to maintain core body temperature. It's simple thermodynamics. It's easier for skinny people to get cold and so skinny people have to burn more calories, thus keeping them skinny. It is a feedback cycle. I'll bet there is a certain threshold of surface area to mass ratio that mus be attained if you are trying to lose (or gain) weight, then after that it become much easier to attain your goal.