Increasing returns to scale and decreasing returns to scale:
Increasing returns to scale occur when increases in all inputs by a certain percentage cause a relatively higher percentage increase in output or total product. For instance when units of capital and labour are doubled (i.e. 100% increase in scale of production) if output more than doubles (say increase by 150%), then the firm is experiencing increasing returns to scale.
On the other hand, decreasing returns to scale occur when the scale of production of a firm is increased by a certain percentage, output or total product increases by less than the given percentage. The total product indeed becomes larger but does so at a lower rate than the rate of growth of all the inputs used in production. for instance, if a 100% rise in scale results in a lower than 100% rise in total product then the firm is experiencing decreasing returns to scale.