Smaller persons lose heat faster because their surface area is greater than their overall volume. So, if they're outside in the sun, they'll heat up more quickly. Skinny people also tend to burn more calories per unit of time because they have more muscle tissue that needs to be heated up to make sure it's working hard. Fat people don't need to work as hard because they can soak up some of the excess energy from the environment. The other factor involved with loss of body heat is how well your skin conducts heat. The darker your skin color, the less heat you will be able to conduct away from your body. However, if you are white or pale colored, you will be able to lose much more heat than a black person would under similar conditions. In conclusion, smaller people will lose heat faster because they have more surface area relative to their volume, and they will also lose heat faster because they have less effective skin color to reduce their temperature. White or pale colored people will still lose heat faster than black people, but by less percentage points than those last two factors would suggest.