Perfectly competitive firms. Economies of scale My textbook says the following:

"Perfectly competitive markets only achieve productive efficiency if you assume that there are no economies of scale in the industry."

Why is this the case? And by "in the industry" does it mean external economies of scale?
Put simply, a competitive market is one with a larger number of smaller firms &mdash; but economies of scale are best exploited by a smaller number of larger firms. 

This doesn’t mean that a competitive market can’t take advantage of economies of scale, just that such a market is only guaranteed to reach productive efficiency if the size of the firm isn’t a factor.