There is another force in play: the degree to which support makes the individual more powerful. I am much closer to my full potential as a problem solver when surrounded by effective management/facilities/IT/legal/differently-specialized-peers/enough-people-to-work-on-truly-large-systems than when I go it alone. Having spent significant time both as a freelancer and in mind bogglingly huge corporations, the corp lifestyle has its drawbacks for sure -- but I myself find the benefits of support and large problems to dramatically outweigh the drawbacks of politics and lack of freedom. Your mileage may vary, of course. Shoot, my mileage has varied over the course of a career. Still, the intuition seems solid: you aren't going to be solving Google-datacenter or Amazon-logistics levels of problems with your personal startup calendar app. At least, not usually, not for a while.
If there is a minimum size market which can maximize the impact of a good developer, I suspect there might be a minimum size company, too.