I think a lot of the footwork as well. MMA fighters have to be ready for many things and need to also be able to throw more than punches, and move differently because of that. boxers optimise their footwork and movement to use everything to drive their punches.
although i find it hilarious how different fighters have different physiques, like wilders tiny legs yet the fucking rotation from his hips and core and then the way he just launches his shoulder and back into those punches ouch. i guess you get a lot of weirdly powerful phsqiues, like long and thin dudes like thomas hearns or short and just big legs big wrists big hands dudes like tyson.
ngannou is just big and strong, but still very fucking strong.