Well, classrooms are of limited size. I know that the classrooms at my old high school were only designed for thirty kids each. Now they hold nearly forty each. There is a significant cost from having correspondingly less space per person. The corresponding reductions in mobility and classroom flexibility have an impact on learning.

This is especially pronounced in science labs. Having even one more person per lab station can have a surprisingly detrimental impact on learning. If there are two or three people at a lab station, then pretty much everyone is forced to participate (and learn) in order to finish the lesson. However, if there are four or more kids at a lab station, then you can have a person slacking off, not doing much and the others can cover for the slacker. The slacker doesn't learn anything, and the other students are resentful because three are doing the work of four.