I'm curious why YC likes 2-3 founders over 4-5. Do more people = more potential for internal drama? Or are larger groups more resistant to changing on the fly? I would think that 4-5 talented go-getters would be better than 2-3.
Another theory I've heard is that a startup does as well as its worst founder, and the bigger the group, the more chance you have someone who's no good.
The fundamental problem comes from a form of information asymmetry where any individual is not able to assess the amount of labor placed by other members. If a project is run by 5 people and requires labor input of 5L, an optimal allocation of be L from each person. However, because each person does not know how much labor other people are placing into the project, they assume that others might put more than L, or the total labor from others are 5L-x where x<L. Hence the person in question only puts x amount of labor in the project. Alas, if everybody thinks in the same way, we only have 5x, which is far less than the required 5L. Unless all 5 people are together at all times, there are bound to be information asymmetry, and it is just much easier to manage with fewer people, in the case of 2-3.
(Obviously there are exceptions. I am a co-founder of one of those exceptions, since Justin.tv has 4 founders. But it's a good rule.)