The ancestors of horses are wild horses. They are large grassland dwellers with long legs, high head and big chin, which enables them to run and breathe enough air. As time goes on, the horse becomes bigger, its legs become longer and its head becomes smaller, which is beneficial to running and breathing faster.
On the contrary, the ancestors of donkeys were short grassland dwellers with short legs, low head and small chin. Although donkeys are smaller than horses, their genitals are smaller and their whips are shorter because of their biological characteristics and evolutionary history.
In a word, the reason why the horse is bigger than the donkey and the reason why the whip is shorter than the donkey is mainly related to the biological characteristics and evolutionary history of the horse.