Weight classes were developed because you have a massive disadvantage being 130lbs vs 250lbs.
Bigger means more weight to put behind a punch, bigger does not mean harder to knock out. A small guy can knock out a big guy with a weak neck, the same way a bigger guy could knock out a smaller guy with a strong neck, simply put more weight = more power = more damage. A good fighter will strengthen their neck and trapezius muscles to support their head when they take big shots and not get knocked out.
As many have said over the years "it's the punches you don't see that do the damage". Anyone can be knocked out, anyone can be caught, it's just a matter of how they stop their brain bouncing around like a pingpong ball in their skull.
You want to increase your ability to absorb hits? neck conditioning and strength.