Why would a percentage based fine make people unequal in the eyes of the law? The percentage does not change and everyone would feel the punishment in the same

Why would a percentage based fine make people unequal in the eyes of the law? The percentage does not change and everyone would feel the punishment in the same fashion If a baby were to punch another baby, the second baby would feel pain but if that same baby were to punch a full grown 94kg man, the man would feel nothing. So we scale up the situation appropriately to two 94kg men. From my perspective, having a small fine only hurts people who are poor where that is not even thought of as a punishment of even an inconvenience for the rich.