"Pick on someone your own size" is a frequent feeling of children when an adult uses his or her power.
It seems to be a universal response of human beings at any age to feel deeply resentful and angry toward someone on whom they are, to a greater or lesser degree, dependent for providing gratification of their needs. Most people don't respond favorably to those who hold power to dispense or withhold rewards. They resent the fact that someone else controls the means for satisfying their needs. They wish they themselves were in control. Also, most people crave this independence because it is risky to depend on another. There is the risk that the person on whom one is dependent will turn out to be less than dependable--unfair, prejudiced, inconsistent, unreasonable; or the person with the power may demand conformity to her own values and standards as a price of her rewards.
This is why employees of highly paternalistic employers--those who are generous in giving "benefits" and "bonuses" (on condition that the employees will gratefully acced to management's efforts to control by authority) are frequently resentful and hostile "to the hand that feeds them." Historians of industrial relations have pointed out that some of the most violent strikes hit companies where the management had been "benevolently paternalistic." This is also why the policy of a "have" nation giving handouts to a "have-not" nation so frequently results in the dependent nation's hostility toward the stronger one, much to the consternation of the "giver."