Skill, power, conditioning and to a lessor extend size and strength. If you have ever rolled on the mats in submission wrestling you can see big guy after big guy get submitted by guys with 14" arms. Same goes for boxing. I was the buff 23 year old bodybuilder who wanted to box back in the day. I went into the gym with a guy that looked like he never lifted but he was an amateur fighter. Got my ass handed to me. He punched like a mule with a lot of power. That's when I learned it's better to go for a run than do 20 sets of biceps. I learned from that and I ended four and one in boxing when I realize that getting hit in the face hurts and quit. I bet that jui jitsu guy in the video could beat the top ten Olympia contenders this year. It's just the truth. It is apples to oranges though.
Yeah, because he was so dominant against a guy only 200 pounds, right?

All you guys seem to think "rolling on the mats" or boxing in a ring is the same as an on-the-spot street fight.
It isn't. Not by a long shot.
I have both formal fight training and real-fight experience, and I can tell you, the two rarely connect in any real way.
If you're very large and very strong and powerful, on the street you're that much more formidable.