This can be explained by Newton's third law of motion which states that for every action (force), there is an opposite and equal reaction. In other words, when two objects or people come in contact, the magnitude of force which they exert on each other is equal and they both feel an equal reaction force.
It doesn't matter whether one of the colliding bodies is of bigger mass than the other. This Newton's third law of motion is also known as the law of action and reaction.
