Economies of scale
Economies of scale are the cost advantages exploited by expanding the scale of production in the long run. The effect is to reduce long run average costs over a range of output.

These lower costs represent an improvement in productive efficiency and can feed through to consumers in lower prices. But economies of scale also give a business a competitive advantage in the market-place. They lead to lower prices and higher profits!

Increasing Returns to Scale
Much of the new thinking in economics focuses on the increasing returns to scale available to a company growing in size in the long run. If a business can sell more output, it may become progressively easier to sell even more output and reap the benefits of large-scale production.
An example of this is the computer software business. The overhead costs of developing new software programs are huge - often running into hundreds of millions of dollars or pounds - but the marginal cost of producing additional copies of the product for sale in the market is close to zero. If a company can establish itself in the market in providing a piece of software, positive feedback from consumers will expand the customer base, raise demand and encourage the firm to increase production. Because the marginal cost of production is so low, the extra output reduces average costs, giving the business the scope to exploit economies of size. Lower costs normally mean higher profits and increasing financial returns for the shareholders of a business.

The long run average cost curve
The LRAC curve or ‘envelope curve’ is drawn on the assumption of their being an infinite number of plant sizes – hence its smooth appearance. The points of tangency between LRAC and SRAC curves do not occur at the minimum points of the SRAC curves except at the point where the minimum efficient scale (MES) is achieved.
If LRAC is falling when output is increasing then the firm is experiencing economies of scale.  For example a doubling of factor inputs in the production process might lead to a more than doubling of output leading to increasing returns to scale.
Conversely, When LRAC rises, the firm experiences diseconomies of scale, and, If LRAC is constant, then the firm is experiencing constant returns to scale.

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There are many different types of economy of scale. Depending on the characteristics of an industry or market, some are more important than others.
Internal economies of scale (IEoS)