What is the relevance of profit per employee to the question of the power of organizations?

Corporations exist, if they have any purpose at all, to maximize profit.

For the owners and shareholders though, not for the employees, unless they are all partners. As to why more employees could lead to lower profit per employee. Suppose a smart person running a one-man company hires a delivery truck driver. I'd expect it to happen there. That's only an example but I think it suggests some hypotheses.