[cryptica]

It's supply and demand. A higher supply of people means that each person has lower value. You get a similar effect if you move to a big city after having lived in a small or medium sized town. People who move to big cities voluntarily turn themselves into commodities; they end up renting tiny apartments which cost a lot and can barely afford to save anything and they can be quickly fired from their jobs due to high competition.

As population size increases, the baseline value of a human being goes down. The baseline value of a human being is the value of the person after the value of their skills and capital have been factored out.