Nutrition in childhood, especially early childhood, is extremely critical to development. The average person historically was remarkably short, generally because of childhood malnourishment. Simply being properly nourished in childhood would have led to a huge height and weight advantage (also probably considerable advantages in brain development). Even into the 18th century the average height of an officer in the British military was about 8in more than that of the enlisted men.

On top of that a knight would have eaten a lot more fat, meat, and vegetables than a peasant, who would have lived almost entirely on bread. This wouldn’t have left the peasant totally protein-starved because it was bread made from whole grains, but a knight would have been getting much better returns on their exercise throughout life due to their more varied diet.