Belly fat doesn't automatically mean someone is unfit - it could actually act as a reliable reserve of energy during long and uncertain military expeditions, or times of drought that were much more prevalent in earlier times. And there's nothing to suggest they aren't rugged and hardy underneath all that flab. It's actually the 'six-packers' of the modern era of gyms and 'lean' diets, who are physiological oddities from an evolutionary/historical standpoint - even lean and athletic youth typically had a flat belly with a faint "groove" running down their midriff at most