One of Malcolm Gladwell's books made a convincing argument that reducing class size quickly runs into the issue of diminishing returns. A cursory Googling suggests there's plenty of research to back it up[0][1]
The effectiveness of in-person learning may have more to do with social interaction and peer motivation. A teacher with about 20 students who is able to create a general excitement for learning in the class seems to be the most effective. A classroom where the "spirit" of learning is alive is ideal for a lot of kids.
[0]https://edcentral.uk/reading-list/431-what-we-have-learned-a... [1]http://hanushek.stanford.edu/publications/evidence-class-siz...