Smaller guys tend to be much more agile, fast, flexible and have a higher strength/weight ratio.
So even if you manage to take them down, it's harder to keep them down. They probably can bench press each other off.
Instead, at HW, there's a lot of clumsy people and it's difficult to escape from the bottom. This alleviates as the weight goes down.
Maia sure has a great single, but his greatest assets are:
1) he will keep you down after, with a big threat of submission.
2) if he fails he has a backup plan of pulling half guard and threaten to sweep and he's amazing at it.
3) he improved his striking a lot. Of course it's not his forte and he now avoids risks, but he has no issues landing straights on the opponents who focus just on defending takedowns. So whatever people may say, Maia is big at 170 and nobody likes getting punched in the face by him, so this helps him to set up his single.
I'd say the best takedown for mma is any that does not land you in guard. Because not everyone has Maia or Khabib level passing.