פירוש על קידושין 50:3

According to Rabbi Meir and Rabbi Elazar large animals are acquired by being “handed over” from their owner to the purchaser. This would entail the purchaser taking hole of the reins, saddle or even hair of the animal. Small animals, however, may be lifted and therefore they are acquired only through lifting. They are not acquired by being handed over or by being led.
The sages disagree with regard to the acquisition of small animals. Despite the fact that they can be lifted, it is still difficult to do so. Therefore they are acquired by being led. Since goats and sheep do not have reins they are led by being pushed or directed by voice commands.