Cooper's fat?!

I was helping Cooper get ready this morning... I pulled out some clothes for him to wear. He always has an opinion about the shirt I choose. He didn't like the one I picked today and when I asked why he said, "It makes me look fat!" 

I said, "That's just silly because you're not fat."

Cooper, "I know that! giggle giggle"

He picked out another shirt. 

... as a parent we always wonder if we're messing up our kids. I make it a point not to say stuff like that around them. Seriously?! Where does it come from?