How does a low-cost elasticity represent its diminishing returns?

Low cost elasticity means that total cost of production does not increase much when there is an increase in a unit of output. When a product has low cost elasticity, it is profitable to produce more units of output. As the quantity of output keeps on increasing, there comes a stage when the efficiency of producing one more unit of output decreases. Addition of one more input increases output at a decreasing rate. The marginal productivity of the input starts diminishing. This marks the stage of diminishing returns.