The amount of value in the economy never changes. Our perception of value is X and all movements of capitalism are to establish equilibrium with X. It is through this myopic rush to stabilize profitability that things like the profit rate drop, wages get squeezed, productivity goes up, etc etc etc when people try to use the government to act on, say, the min wage. Because capital is constantly trying to find equilibrium with what current rate of profit.
Making more commodities means producing above the socially necessary labor time . (The time it takes on average to produce a particular commodity.) So, because you have, again, disturbed the current equilibrium by producing above the socially necessary labor time capital is going to try and stabilize and find the new rate of profit; everyone is going to mad dash to try and meet the new social standard of labor time and thus will raise the average above that of what once was the previous average; You need to understand, also, through this relationship that producing more and doing it more quickly means you are able to lower your prices and, as such, in order to keep up and not die out your competition is going to follow in your coat tails.
It is through this process that the rate of profit or the ability to exploit surplus labor falls because when you raise the bar everyone follows suit, cutting their bottom line and creating a new standard over and over and over and over again.
That is why making more commodities means less over all surplus labor value for the capitalists because everyone is constantly trying to sell their product for lower and lower in a myopic dash to make more profits while simultaneously cutting labor through innovation and increasing productivity.