Thing is, the top group is Blackpink, but they barely make comebacks and when they do they would surpass the million, but the other "top kpop girlgroups" like Twice or Aespa sell around 600k which is quite a gap. You have to take on account that while groups debuting on big companies are selling a lot right now, if they start to debut more and more groups and to oversaturate the industry even more, I don't know if it's realistic to expect all groups from big companies to sell that much since debut, we would end up calling nugus almost all groups from mid-tier companies. Nugu for me carries the meaning of "pretty much nobody knows them/they are barely making it", so maybe it would be better if you consider those groups low tier if anything, because actual nugus can't compare at all to the ones I mentioned.
I don't think we shouldn't compare 3rd gen groups to 4th gen groups, the same way you wouldn't compare 2nd groups to 3rd group sales. Everyone is benefiting from kpop growth but especially new groups because people tend to stan new groups more than old ones (3rd gen groups) with a few of exceptions like bts and bp.
"I don't know if it's realistic to expect all groups from big companies to sell that much since debut" I'm sure ygngg will be able to sell 200K on debut, hybe groups will be iland another survival show and sakura group, they will sell a lot too. I don't know how well adore will do on debut tho. I can see the most of top groups reaching 700K-1M sales in the next few years, that will become the average like the average sales for top bgs is 1M right now but I could be wrong and I'm overestimating kpop sales growth.