It's a lot easier to hide how thin you are, by wearing clothing, than how overweight you are. Also, I find that "skinny" has had an evolving meaning as the average size of America grows larger, to the point where slightly underweight people are now considered "skinny," where as before, they would be considered lean, etc.
not too skinny the person has to have at least a little meat on em, if they have like chicken legs, ew
too skinny is creepy. It's like hugging a stick. They have to fit just right, and sometimes that needs a little meat on the bones