i don't think its genetics its a massive sample size and genetically they are not too dissimilair to the countries around them plus they have a large migrant population.
Well the countries around them are the tallest in the world so I wouldn't throw that genetic theory you have out the window just yet. The top 4 are all countries all border each other.
Country Average Male Height Population
Netherlands 182.5 cm 16,318,199
Denmark 181.5 cm 5,413,392
Switzerland 180.5 cm 7,450,867
Germany 180.2 cm 82,424,609
Croatia 180.0 cm 4,496,869
Norway 180.0 cm 137,253,133
Sweden 179.6 cm 8,986,400
Iran 178.7 cm 67,503,205
Austria 178.2 cm 8,174,762
Finland 178.2 cm 5,214,512