Obviously it’s not just size but a mix of size, strength, athleticism, resilience, etc.
So if we are talking about elite strongmen competitors, therefore not just random “big guys” but absolute outliers in all the above parameters, IMO pro fighters under 200 pounds would have big problems and under 170-180 would almost certainly lose.
Elite strongmen can literally pick up and toss super awkward 400 pounds stones … if they grab hold of a 170-180 pounds man with bad intentions, it’s not going to be good.
I think a lot of the sparring footage we see is “compromised” because it’s kind of like an adult training with a child, they have to be careful not to accidentally hurt them just by awkwardly landing on them or cranking something with a tiny bit too much strength. Tangling with a guy like that just for fun can pop a shoulder or tweak a knee just by falling the wrong way.