I'm pretty sure that for me it depends on the situation. And I think that is where standard education runs amok. Kids are not blank forms to be molded in an automated education box. But putting between 15 to 30 same-aged students in one room together and expecting that they will always learn their best makes no intuitive sense.
There are times that I need a group and the group helps me along and fills in the areas that I can't figure out on my own. And other times I like watching others to see what their work and progress looks like.
There are other times when I'm learning that I need everything to be just right. I have to have a whole area to myself, with no noise, with a cup of coffee ready by my side and the temperature just right.
Part of my eventual success in university was figuring this out, but it took un-learning my habits from school to figure out how I could learn.
So, the answer to the riddle who does class size the best, public, private, microschools, or homeschoolers? I'd say none of us are doing it right all the time because to figure that out the students need to be able to tell us what works for them. And that is part of our job is helping students figure out how they learn best and how to recognize that within themselves. It's our goal to help our young students learn that skill now so they don't have to struggle as long as some of the rest of have.