This is dogma, not proven fact, and most people that argue this tend to use self-serving metrics and a tailored definition of "efficient". Some counterexamples: early Google was much more efficient in responding to market changes than the current top-heavy organization; small hospitals tend to have better health outcomes (both per patient and per dollar) than large chains. Tesla was able to innovate much faster than established behemoths.
I think you mean "nimble", "versatile", or "agile". None of these imply efficiency in the same sense economy of scale does (ie cost to produce a single deliverable unit).
There are good examples, though—you can produce a single gold ring a lot cheaper than you can produce a one-of-a-trillion of them, cuz at some point you simply run out of gold. Another example is running into a cap in demand. Classic sigmoid vs exponential patterns.
This is dogma, not proven fact, and most people that argue this tend to use self-serving metrics and a tailored definition of "efficient". Some counterexamples: early Google was much more efficient in responding to market changes than the current top-heavy organization; small hospitals tend to have better health outcomes (both per patient and per dollar) than large chains. Tesla was able to innovate much faster than established behemoths.