Resource limitations.

There is only a limited amount of any given resource available. Decreasing the number of people therefore increases the amount of resource available per person.

There is a point at which decreasing the population will begin decreasing average utility, but to me it seems nigh certain that that point is significantly below the current population.
I could be wrong, and if I am wrong I would like to know.

Do you feel that the current population is optimum, below optimum, or above optimum?