this is a bit of a myth. It was not uncommon for people in the Middle Ages to be tall - even skeletons as far back as Anglo-Saxon days are tall. It wasn't till the industrial revolution, when people started cramming into insanitary cities and eating scraps, that they became shorter. And royalty, being pretty well looked-after medically (then as now) were likely to be the healthiest people in the country.
Some people argue that the low doorways in old houses prove the inhabitants must hae been short. But it's more likely they just made the doors short to keep the heat in, and stooped when they entered.