Tall people

I think the reason is diet. People in northern climates tend to eat a more varied and healthy diet than those in warmer climes. This is especially true in the early years because parents are economically better off and take better care of their children. Those in warmer climates tend to be poorer and spend more time working and do not have the means to afford to give their children healthy foods nor can they afford more attentive medical care.

The food theory sounds good, but the fact that people in northern countries have more money and though a better diet is relatively new and has a lot to do with transportation of food and other goods which is only happening in the last 100-200 years (before the invention of train, moving foods in ships took ages and was limited to dried good.

It take many generations for a whole nation to develop evolutionary, way way more then 100-200 years.

I actually think it's the other way around - the northern countries were very hard to live in in old times, since it's very hard to make any food grow in a cold weather, most of their diet was based on hunting - for that you got to be strong. The tall fat body obtain more fat and though survive harsh winters.

It in my belief that shorted smaller people die at a younget age and people with a larger physique  tend to have a better shot at reaching adulthood and reproducing.