The "natural" size of a company is determined by the relationship between transaction costs and the costs of organization. See this seminal paper for a fuller exploration of the topic: http://onlinelibrary.wiley.com/doi/10.1111/j.1468-0335.1937....

I would not be surprised, though, if there is a sudden increase in organizational costs above a certain threshold due to the limitations of human social capacities.

It might be the natural size optimal for efficiency but not for survival (in the natural selection sense). The capacity of an organization grows with headcount even with diminishing efficiency per capita.