As a former teacher, I think it doesn't take a genius to see that smaller classes, especially for younger children, equa;s better teaching and learning.
When kids are older and doing subjects where they get information from texts and lectures, it may not matter as much, although I think you could make a case for more personal attention there as well. I remember some college classes where I was one of a couple hundred and the professor wouldn't have recognized me if he met me on the street. Not sure that was a good system either but those of us who were motivated did OK.
But when you are teaching things like reading and math skills, you need to attend to each child individually. They don't all learn in the same way or at the same pace. ITheoretically, if you have 30 kids in a room, they each get only half as much individual attention as when there are 15. What actually happens is, the teacher spends most of her time struggling with disciplining those who won't comply and trying to meet the needs of the lowest achieving students (who should actually be getting an aide or special classes) and those in the middle or on the high end and those who are quiet and docile get nothing. That is one reason we have a lot of kids who never master the basic skills. There are only 24 hours in a day and kids are at school usually no more than 5 to 7 of those. Take out time for music, art, PE, lunch, recess, etc. and there is very little class time yet. By the way, I'm not complaining about those extra things: there is research to show that kids who have things like music and art do better in reading and math. And PE and recess are essential, although right now there is a trend toward eliminating recess so kids can spend more time in class. Yeah, that works really well for all kids, especially those with ADHD and some issues like that, right?