One thing to keep in mind when imagining "Knight vs Peasant" combat is that it's not like they would be fighting in some sort of idealized battle arena. Knights have horses and peasants do not, which means that barring exceptional circumstances, the knight has the initiative to determine when and where combat happens. People without horses have an extremely difficult time fighting people with horses, beyond just the reality of actual combat. If the horsemen want to raid your supply lines or burn down your countryside or disrupt your trade networks, how do you stop them? They can concentrate forces much more quickly than a foot army can form or react. In the 11th century a few hundred Norman knights conquered Sicily and southern Italy. Even through the 1860s a few thousand Comanche controlled a large portion of the American southwest because there was no good way to fight them.