Probably a collection of factors including genetic, dietary and environment factors.
How long a civilization has been settled and sustain on agriculture I think also has a factor (you look at for example Chinese/Japanese, Mediterraneans and Middle-Easterns they are generally not tall or heavy as vikings, people of the steppes or Africans, probably because of the fact that they have ready access to food through ease of farming and don't need to store so much energy any rely on hunting).
How long a civilization has been settled and sustain on agriculture I think also has a factor (you look at for example Chinese/Japanese, Mediterraneans and Middle-Easterns they are generally not tall or heavy as vikings, people of the steppes or Africans, probably because of the fact that they have ready access to food through ease of farming and don't need to store so much energy any rely on hunting).