If there's one thing I've learned from watching lots of UFC/MMA - it's that it is one of the most exciting sports because anyone can beat anyone. Why? knocking someone out is just one good punch away.
If this guy had a natural talent for clocking people on exactly right spot: on their jaw line with a great straight striking technique or accurate haymaker, or temple again with a swinging haymaker: then of course he could floor people in an instant.
Power, size and weight count for sure. But great technique can also surpass any of these.
Strength generally isn't a factor, but weight can be. And everyone has a different body type which changes their weight transfer. When you swing a limb you simply have more weight force behind it due to the weight of the rest of your body. If you can even half throw a punch properly, that weight is transferring through the fist/hand.
Precision and accuracy matter a lot! Getting someone in those right spots can floor someone in an instant.
They stop fights as soon as this happens because that person is in an incredibly vulnerable position as soon as they get knocked down. Their brain has switched off for a moment to protect itself, and now they have no control for protection (even some flailing arms in the way can counter a lot more damage) and their brain state is also just very vulnerable to more permanent damages.
I have sparred opponents in training MMA/boxing and there is a very different feeling between taking heavy shots in areas that can take it (arms, chest, stomach to a degree) if you're protecting properly.
But whenever I've been cracked on the chin in the right way, even without much power and just good timing, I've been literally spun around in place from the effect. You notice exactly what its like to get caught right on the chin just once vs receiving a lot of blows whilst hands up and protected.