One of my old dutch colleagues that I used to work with in Spain, sent a group of us an article on The Times website entitled "The Dutch get ahead for heights". It's all about how the average height of Europeans has surpassed that of the US, who always dominated the world of tall people. Particularly, it highlights the Dutch as being the tallest in the world now and quotes:

Professor Komlos suspects that it may have something to do with the junk food trend that hit the US before Europe. Alternatively, the more equitable healthcare systems of Western Europe may enable more people to reach their optimum height, while many poorer children in the US remain deprived.

But these explanations are conjecture, he admits.

I think this scientist may be onto something, however he hasn't considered evolution. If we take into consideration the potential effects global warming will have on the Netherlands, I guess they have to be the tallest to keep their heads above water. :-)