I'm not sure why the Dutch are so tall, but I think they always have been haven't they? Dutch and Scandinavians. Must be diet related, but they are clearly taller than their French and German neighbours too, so something is going on.

Probably impacted by Immigration too. The top three non Dutch national backgrounds in Netherlands are Turks, Moroccans and Surinamese, the average height for them being 169.1cm, 168.8cm and 168.4cm. In UK it's Indian, Pakistani and Bangladeshi, the average height for them being 170.8cm, 161cm and 158.7cm according to Wikipedia anyway.

Obviously over time this changes, but it would likely take a few generations.