Economies of scale basically. If company A serves 100 000 customers and needs $1m to do R&D of a particular feature, and company B is much larger, serves 1 000 000 customers and due to organizational inefficiencies needs to spend $2m of R&D man-months to do the same feature, from the customer's viewpoint they're still much more efficient than company A, since they have less expenses per customer, so they can either charge lower prices or afford to add more features to their product.
It's like the efficiency of Amazon and Walmart - a small store can outcompete them on some aspects like, say, customer service, but not on efficiency as measured as cost of overhead per item or sales dollar. Lots and lots of good things are prohibitively expensive unless you can spread that fixed cost among many customers.
But economies of scale are not efficient for making money. It's about moving more units at thinner margins. More volume has to be moved to make a buck.
Economies of scale are efficient for unit cost, for sure; the production of stuff is efficient.
Two guys laying residential bathroom tiles are more efficient at making money than Amazon, in some sense, though.
Efficient in what parameter(s), compared to the service and money extraction performance of small companies?
Big companies extract an absolute bigger volume of money and provide a bigger volume of service; that is undeniable.
But a measure of some sort efficiency need some sort of denominator under that.
E.g. from the point of view of a top brass executive getting rich, a bigger company is certainly much more efficient than a small one.