As I understand it, the size of shire horses ties in with the military side. They had to breed big horses to carry all those heavily armoured knights, along with the horse armour.

Apparently, big horses don't fare so well in cold climates, as the Germans found out when General Winter struck the Wehrmacht in 1941. Scrawny little Russian ponies, however, survived quite nicely. That might be why the fully mailed knight was a western European thing.