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?

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?