Humans have a greater degree of self-awarness, goals, values and dignity. I'm not saying some animals don't possess some version of these things - I think my dogs know that they are alive, after all - but I am saying that humans have a higher DEGREE of self-awareness and dreams for the future. I don't think many of you would say that a fly has the same mental capacity as a horse. Well, I say that a human has more mental capacity and self-awareness than any other animal. Therefore, abusing humans is worse, since you are inflicting a greater amount of pain, since humans have a greater emotional and intellectual capacity to experience that pain.
Both are bad - but with humans, it is worse. When you hurt a dog, you are inflicting pain and misery on a creature. When you hurt or abuse a human, you strip them of their dignity, you cause them to question their self-worth and you instill in them psychological terror on an order much greater than that experienced by animals. The abused human can acutely sense the viciousness of their tormentor, can feel the injury much more acutely and probably has a greater memory of the event. While animals can and are scarred by their experiences of abuse, the effect it has is not as great as it is on a human.
Also, an abused animal is a sad sight to see - but animals don't have dreams (as in, goal-oriented dreams), they don't have a complex morality, they don't have the capacity to feel the depths or heights of emotion as we humans do. I've never seen an animal look at the stars and ponder its own existence or its own history or sense of place in the world. Humans can do these things. Thus, our pain is worse than the pain of animals.
I'm not saying that animal can't feel pain - they obviously can. It's just on a lesser scale and creates less psychological torment for them, since their minds are more primitive than ours.