Economies of Scale. A firm experiences EoS when long-run average costs fall as output rises.
A larger firm becomes more productive and lowers average costs in the long-run through economies of scale. Economies of scale makes a firm’s short-run cost curve shift down in the long-run.
Remember the short-run is that period of time in which at least one factor of production is fixed. All factors of production are variable in the long-run.