Medieval War Horses Were Smaller Than Modern-Day Ponies
iflscience.comThey should have mentioned that the average size of people was smaller too. I suspect it all comes down to the amount of food available. Less food equals smaller organism -- both people and horses.
The article's title is misleading. The study is on war horses from Britain and the smaller horses were from the Norman period, not the entire medieval era. For some reason the average size of horses shrunk for a few hundred years in Britain.