Marx has established that an increase in the productivity of labour leads to an increase in the proportion of the value of the commodity product produced in the form of surplus value, and hence in the form in which it can be accumulated, that is, redeployed as new capital, increasing the mass of capital value advanced and therefore increasing the scale of production.

More (pdf, 78 KB): The Chapter on Capital: Part 12: Relative Surplus Value (III): Ricardo (pp. 347-353)