In a nutrition study of baboons it was found that exercise had more to do with obesity than caloric intake. It was also found that some were much more susceptible to obesity than others even with the same amount of exercise and the same amount of calories.

"The implication for humans is some people can get away with indiscretions such as not exercising and will gain a little weight without suffering these serious health consequences. Other people are going to balloon out and get sick with less provocation,"

If you have the genetic tendency to be obese you need to pay much more attention to exercise than others without that tendency.