The basis premise, as stated in the BBC article, is intrinsically flawed. Doing a pseudo-scientific study of obesity based only upon relative Body Mass Indices (BMI) is absurd. BMI is a ratio of height to weight, and has only a passing relationship to body fat content.

Muscle is heavier (per cubic volume) than fat. A very physically-fit person will usually have a higher BMI than one with the same dimensions (neck, chest, waist and hip measurements) who is sedentary. Conversely, if a fit and a sedentary person have the same BMIs, the fit one will generally be slenderer. It is a common phenomena for a person to go on a strenuous exercise program and to gain weight if he doesn't also change his eating habits.