If size doesn't matter, why are there weight divisions in boxing and wrestling?
As the saying goes, the race is not always to the swift, but that is the way to bet. Given equal ability, a bigger man will routinely win the fight.
Why do you think Sam Colt is remembered as the man who made men equal? Before firearms, the larger man almost always won the fight.
The same reason there are timed rounds, gloves to protect the hands, mouthpieces, etc.. IMO, it has more to do with organized crime, specifically gambling, and the need to make an event as equal as possible so you can have people gambling on both sides. That's why they give odds - give good enough odds and someone will bet on a fighter they typically wouldn't bet on. Same reason they cooked up point spreads in football. From a "business" view you can't have 80 or 90% of people betting on a fighter and only 10-20% betting on the other. You'll lose big money that way.
I have seen some martial arts "games" in Korea and China in which there were three rules: No attack to the eyes, no attack to the windpipe, and no attack to the spinal column. I never saw the rules actually enforced but they did exist. The games usually lasted less than 40 seconds and had no weight classes. I've never seen them in the US, probably for liability reasons.
All things being equal, size does matter. But then again, very few things are equal in a street fight where a premium is placed on doing maximum damage as quickly as you can. Costin, as the smaller man, felt the need to strike first. Junta's mistake was not clocking him immediately after Costin threw the first punch.
I'm 6'5" and 230 lbs. I've been challenged many times on the basketball court by smaller men for minor contact that larger men just shrug off. Napoleon complex at work, you just walk away.
Because quick blows to the windpipe are not allowed?