What SMB has the luxury of being notoriously unproductive? Economies of scale are very real and tend to make larger businesses more efficient, it's true, but you'll find that causes SMBs to be lean and mean to remain competitive.
Bigger companies can pay a lot more because they are more productive. And further research has shown its the same pool/type of people at each.
Notwithstanding grey-market limitations, people have their motives for accepting the inefficiency of starting or staying small. Potential, for example.