I wasn't making the argument that small markets offer the best returns for effort. Clearly that can't be true either - just by its definition.
The notion that there are an abundance of large markets, many of which are underserved is quite a simplistic argument.
There must be a reason that they are both a) large, and b) underserved.....probably because the barriers to entry are exceedingly high. Energy markets come to mind. Hard to get larger markets than that, but it is easy to find large subsets of customers that are disgruntled and would gladly switch to a `better service`.
The trick is that every Tom, Dick and Harry can't start an energy company. It is VERY capital intensive, even more labor intensive and heavily regulated.
So, I think if you were to reword that statement to say: "There is an abundance of large markets, many of which are underserved...and easy to reach/service for a single web developer anywhere in the world". I would surely argue that to be false.