The heavier weightclasses have a nuch greater chance of KOing their opponents, and staying undefeated is more immpressive, so it should be something like this to be compareable:
HW: 8
LHW: 10
MW: 12
WW: 16
and so on.
Nooooooooooooo, FW, LW and WW are
faaaaaaaaaaaaaaaaaaar deeper talent pools than LHW and HW, so if you're champ at 145, 155 or 170 for any length of time you're generally facing a significantly higher caliber of challenger than at LHW or HW.
So guys like Aldo and GSP were facing primed beast after primed beast after primed beast, most of whom had solid 3+ win-streaks worth of momentum going into their challenges. You just don't get anything like that kind of consistency of challenger quality at the outer ends of the demographic bell curve.
Casuals forget this because they get sucked in by "name value", but hardcore fans will remember that of Jon Jones' first 6 title fights only 1 was against a challenger who had better than an active 2-win streak at LHW when they fought him. Jon is an awesome fighter, obviously, which is why I stretched down to 8 to include him, but his division was so abysmally shallow they had to feed him two consecutive middleweights smack bang in the middle of his reign. I mean, we include Jon in the 8+ club, but considering that neither Chael nor Vitor had fought a LHW opponent for at least half a decade when they were sent into the Octagon with him, maybe there should be half an asterisk next to that 8. Neither GSP nor Aldo ever had such a luxury in building their 9-defense runs.
Also, a "much greater chance to KO their opponents" works both ways. It means if you're champ you may not be better than the challenger, but you can get away with a "lucky" KO. If you're the FW or WW champ you can't rely on a lucky shot to beat a superior opponent, you have to be legit better pretty much every single time.