They're taller on average because their societies have the best health care and nutrition in the world. Other groups of people would be taller if they had the same type of upbringing that Northern Euro children have. Keep in mind that they were short prior to the 1970s; at that time Americans were the tallest people on Earth because they had the best nutrition due to our vast farmland as well as the fact that WWI and WWII didn't wreak the same havoc on us that it did on Europeans.