Everyone has a chin, except for some rare individuals. The chin is an interesting spot. You can consider it a weak spot in the human body, as it provides good leverage for moving your head back and forth. Knock outs are caused by brain trauma, which can easily result from rocking the head back and forth.
A hard strike to the chin can do exactly that, and it does not take much force for a precise strike to do so. A bigger bone structure will absorb more shock, and make it harder for a strike to move the head. Despite this, size does not increase the ability to take a hard strike to a vulnerable point like the chin as much as you think.