More knights means more horses, and not just more, but stronger ones to carry the weight of knights and their armor. War horses, also known as destriers became the norm for knight mounts.

graph TD
K[Knights] --> |Carry armor| AW[Armor weight]
AW --> |Selective breeding| HS[Horse strength]
K --> |Require| H[Horses]
HS --> |Can carry more| AW

Source: Medieval Technology and Social Change by Lynn White