There is a general attitude in life where the smaller a person is, the less of a threat they pose. Time and time has it been proven again that size is not a massive factor when it comes to untrained vs. untrained as long as the size discrepancy is not huge. It is believed that the thought of size=combat ability comes from the fact that children are much smaller and weaker than adults on average.
Regardless, those with little to no ability for hand to hand combat should not underestimate children or smaller individuals in general. Being big will not make up for abysmal skills, and the embarrassment will never end with the availability of video recording these days.