@Chrona Because weight class is a very easy way to gain an advantage compared to building up skill (weeks vs. years) and if you have the perseverance and a good diet and regimen, the end result comes down to little more than genetics. A subpar heavyweight MMA fighter who hasn't been working on his technique half as much as he should be still has a humongous advantage over a lightweight who's actually really good at what he does, provided the weight comes from properly-built muscle, because of mass, reach, fist size, etc. which isn't surprising considering a heavyweight can weigh literally twice as much as a lightweight.
We romanticize various martial arts and artists, but the raw truth of the matter is that if Mike Tyson and Bruce Lee fought at their respective primes, Tyson would break that little man like a twig. That's not to say Lee would suck at a real fight; dude broke records with his build that are extremely implausible at his weight cless. Just not enough records to transcend the laws of physics.
If you have a friend who's much larger and muscular than you, try punching him in the torso sometime. It'll hurt your hand so much more than it hurts his chest that he probably won't even be offended.