Economies of scale are important but I think the thing is, they hit massively diminishing returns after you get past a critical threshold that is usually industry dependent, but related to the cost of capital to overcome fixed overheads.

For example if you can afford to build a fully automated factory to manufacture your product then you are probably 95% as efficient as a company that can build 10 such factories.

The large companies we have now are exceeding that size by one or more orders of magnitude.