I don't think bigger is necessarily better, to an extent, it's just suited for different conditions. Larger is better in cold environments, where a lower surface-to-volume ratio can conserve heat; but smaller can better in warm climates such as the Mediterranean region because it requires less resources and maybe wasn't selected against. Perhaps, also, Northern Europeans sexually selected for size because it was an indicator that someone could obtain enough food to grow tall and strong in a climate where cold naturally drained calories.