I get it, more number of fighters in the division should bring up the standards higher but still, you can't measure the actual skill levels of different weight classes. It's like apples to oranges, or comparing Jordan era to the Lebron era.Would probably be a part of the reason. IMO it's mainly because there's not too many small guys out there let alone ones that fight. You can find 6' 200lbs dudes everywhere you go (maybe exclude Asia and Mexico). Depending on each one they could range from LW to WW in shape after a cut so naturally those divisions are going to have a steady stream of good fighters.
Every argument people use on why MM's accomplishments are not as great as other GOATs of the different division always boils down to him being the small guy and the divison is talentless, yet Henry Cejudo is the highest decorated amateur wrestler in MMA(gold in real Olympics) , but somehow beating him and shutting down his wrestling technique is less valued compared to something like GSP beating Koschek (one of the more decorated amateur wrestlers).