Janitors and guards don't result in "economies of scale". Those are service jobs, and take as much time to do wether the worker is employed by the the large company or a subcontractor. Hell, the large company is certainly more efficient in dealing with the administrative side of employment. The "economies of scale" come solely from the possibilities of enforcing rougher working conditions (which includes wages).

How can you separate working conditions and efficiency ? Very often, "efficiency" improvals come from transfers from the utility function of the employee and that of the employer - the later is monetarised, unlike the former, and that why it shows up as an "increase in wealth" which it is not.

Un roi sans divertissement est un homme plein de misères