Returns to Scale and Economies of Scale are two quite different concepts.  Returns to Scale relates to the nature of the underlying production technology.  Whereas Economies of Scale refer to the leverage a firm has with its vendors due to its size. 

Consider three auto companies.  One produces 5,000 vehicles per year, the second 50,000, and the third 500,000.  At just twenty vehicles a day, the first company doesn’t have the volume necessary to achieve optimal resource efficiency.  For example, to achieve labor efficiency the company would like to break the assembly into a series of highly specialized tasks.  At such a low volume however, this will either require considerable idle time as workers wait for the next vehicle or for fewer workers to remain busy by completing several tasks on each vehicle.  Either choice compromises efficiency.  The company producing 50,000 vehicles has five times the daily volume.  This allows them to more fully specialize their labor without the corresponding wait time between vehicles.  The resulting cost advantage to the second company comes from Increasing Returns to Scale.

Would the company producing 500,000 vehicles in a single manufacturing plant enjoy even greater efficiency?  What about 5,000,000 vehicles?  Not necessarily.  At some point the company may have captured all possible efficiency gains.  Beyond that, Returns to Scale may simply be Constant or under some circumstances even Decreasing. 

To see this, consider another example, mining.  As the volume of mining of coal increases would you expect the rate of coal extraction to increase.  Or more likely, would it decrease as the easiest seams were tapped first?  In this case, we’d expect to see Decreasing Returns to Scale.  As the world mines more coal (or whatever mineral), the difficulty of maintaining production will increase.

Finally, it’s also possible that scale won’t impact efficiency either way.  If one telemarketer can phone thirty households per hour, two telemarketers can phone sixty, and three ninety.  There are neither efficiencies from specialization nor inefficiencies due to constrained resources.   A doubling of scale simply doubles output at double the cost – Constant Returns to Scale.

Economies of Scale is very different idea.  The automobile industry is subject to Increasing Returns to Scale.  But it also benefits from Economies of Scale.  General Motors sold nearly eight million vehicles in 2019.  Most engines hold about five liters of antifreeze.  And so General Motors buys around forty million liters of antifreeze per year.  Imagine the buying leverage this gives them when negotiating with suppliers. 

Economies of Scale refers to the buying power that comes with high volume buying.  And so, while Increasing Returns to Scale and Economies of Scale may be coincident, they’re not the same thing.