There is no doubt that the topic of size and combat ability has been beaten to death. Yes, weight classes exist for a reason, but it’s a different matter in the streets. Altercations in the streets are quick and between individuals with varying base levels in terms of skill, and also varying in size.
For untrained individuals, their chins have never been tested. Those who have training and are experienced in sparring are more than likely to be used to taking hits. While your chin cannot be trained in a sense, there is a large difference between those who are trained to take hits than those who have never taken a hit. Bigger individuals are able to absorb more shock, but a properly landed strike in a vulnerable point such as the chin will still rock them none the less.