1. It's easier to form friendships in a smaller group. In larger groups it is much easier for it to became a wall of people rather than individual persons. Large groups can be extremely overwhelming for children (they are still overwhelming for many adults).
2. It is much easier for the teacher to see the group dynamics, and jump in to make sure nobody is excluded. If the teacher doesn't do this, many of the benefits of smaller class size will be lost. Teachers need to very vigilant to teach that it's not okay to exclude kids and not play with them, its not ok to bully, etc. If there is nobody in the room to foster healthy relationships, they will never reach even close to their full potential.
I have seen and talked about this in quite a bit of detail with educators who are continually successful, and it is not hard to figure out - it requires devotion rather than advanced pedagogical theory and strategy.
how would small class sizes help make children less self centered?
more attention from the teacher? why would that help?
i have to admit i have no clue what factors help kids be less self centered.