The Little Book That Builds Wealth - Chapter7

In this chapter the author explains about the size advantage.  Companies that can spread their fixed cost on a larger base have significant advantages than other companies and this provides a very strong moat as others would have a high fixed cost and therefore cannot compete with this company.
Economies of scale can be in three categories viz distribution, manufacturing and niche markets.  Large distribution networks provide very high competitive advantage as incremental cost as lesser but the value increases manifold.   A new player would have to make very high investment and time to replicate the distribution network of large established players.  Unless they have deep pockets and patience, they would not do it and this provides the advantage.
In manufacturing larger the facility and more the capacity, fixed costs get distributed and the cost becomes lower.
Final type of scale advantage is being a dominant player in niche market.  Even very small company can be a dominant player in a niche market.  Basically a big fish in a small pond is better than big fish in a big pond.