Someone who is so fat, that they can be compared to a skinny whale.
"Look how fat that kid his."
"Ya, he's so fat, that he's an anorexic whale!"
Someone who is so skinny, they can be compared to a whale as an opposite, thus the use of the word anorexic.
Seeming to be opposites, that's what they describe, someone so skinny they are as skinny as a whale is huge.
Apples: You never bring a lunch, you never eat anything, and you weigh forty pounds! Eat you anorexic whale!
Berries: Umm. I weigh 44 pounds