so why there are no heavyweight champs who weigh 80 kg? 120 kg trained fighter will always be more dangerous than 93 kg or 80 kg. you guys are talking out of your ass.
Fighting in the ring according to sporting rules you have to pace yourself to last 12 rounds and have to deal with damage over a long time, you're also fighting with gloves and your movement is limited by the ring and you're not allowed to use attacks to the joints, knees, eyes, groin or throat or attacks designed to kill.
Rocky Marciano, the only undefeated heavyweight champion weight was 85 kg's at 5'10 and he defeated fighters considerably heavier and taller than him.
In Thailand and Burma they never used to have weight categories until relatively recently, before that they fought bare knuckle with no rounds and no ring.
In Burma they still fight bare knuckle and can head-butt and mostly don't have weight classes.
Also, for at least the past 6000 years people have generally used weapons to fight each other in real life.