This is a bit correllation / causation. Larger companies aren't better in themselves, otherwise state monopolies would be the best answer. Developing countries have lots of businesses sure. But "size" is not a problem itself. It's usually industry (e.g. lots of street vendors because it's the easiest way to make a living if you have nothing else) and lack of capital investment (e.g. people hammering steel by hand rather than having some machinery)
A lot of capital investments only makes sense at scale, though. The machines are more productive when manned 24/7, and the machines are often more efficient the larger they become.
Economies of scale are real, but there are also diseconomies of scale. Machines are often more efficient the larger they are, but organizations tend to become less efficient the larger they get!
