The thing is, it's more of an observation with a child...they don't understand the negative connotations associated with things like different skin colors, body shapes, age.
I will always remember when I was a kid that my aunt, who is overweight, said "How you doing, Slim Jim?" and I responded, "Just fine, Fat Matt!" I thought I was being clever in rhyming back to her and thought nothing of it, but her face crumpled and she nearly cried. My mom chewed me out for it and I felt like such an awful person. I never meant to hurt her feelings at all.
Like you said, kids don't really think of things like that as being hurtful, but I don't understand how adults could be so immature as to continue hurting people's feelings unnecessarily just for the sake of doing it. It's a real shame.
