I'm not sure it's fully from efficiency of scale, especially outside of manufacturing. What I keep seeing is using one's size to keep out newcomers as the most important factor. You have more patents and more lawyers and can accept financial losses longer on specific products to keep them out: products A and B subsidize C when C is threatened. And you can bundle multiple products into "special deals" the way a single-product producer can't. And there's the network effect that social networks use: you need more customers before you can get customers.

In short, economic bullying.

Since the early 80's I've watched Microsoft grow, and don't see it about raw merit at all. It's poker, not chess.