The welfare state seduces individuals into behavior patterns that exploit this survival trait, but at the cost of imposing extra burdens on the productive individuals. The more generous the welfare benefits, the larger the number of people that will be drawn into this parasitic mode of existence. At some point, the burden of the parasitic portion of the population may overwhelm the output capacity of the productive portion. Thus, it is the ratio of parasitic to productive individuals that is crucial, not the total size of the population. Absent parasite-inducing paternalism, a larger population could offer significant advantages.
More people means more minds.
Having more minds working on human problems improves the chances of finding solutions. There is more opportunity for specialization and the depth of expertise that specialization brings. The dramatic acceleration in science and technology in our high population era is evidence for the potential advantages of a growing population. On balance, the environment is getting better. Consider the case of transportation.