>These are just assertions. Efficient compared to what?
Compared to numerous small companies of the aggregate same size. It's not just an assertion, Google (and other big companies) produces incredibly high rates of value per employee and goods at extremely low costs to consumers.
>Why should an organization even have 180000 employees? What determines the distribution of size of organizational units observed in an economy?
Coase told us the answer to this[1]. Organizations are going to be as large as they can possibly be until the internal cost of organization is larger than the external costs of transaction with other organizations. How large that is depends on the tools available to organize and the quality of organization, but tends larger over time because management techniques and information sharing tools become more sophisticated.
The reason why large organizations are efficient is obvious if you turn it on its head. If we were all single individual organizations billing each other invoices we'd have maximum transaction costs and overhead. Bureaucracy and hierarchies minimize this overhead by turning it into a dedicated disciplines and rationalize that process. A city of 5 million people, centrally administered produces more economic value than a thousand villages with the same aggregate population.
[1] https://onlinelibrary.wiley.com/doi/10.1111/j.1468-0335.1937...