If considering what was diet of "irregulars" - peasants in comparison to knights than they should be "bigger" than peasants. On the other hand just their gear should make them so.
I remember that in Golden Horde game it was big difference even in size among those different levels of troops. When I zoomed bigger battle it was nice to see visually some "giants" among "regular" soldiers. (though I did not like that huge difference like in Warcraft III for example where heroes were too big among the rest of army)