Maybe… but they do very well, just look at Norway and Sweden; very small country, they will not be the next superpower, but still they are doing very well from an economical, ecological and social point of view.
What I will say is that smaller countries (in terms of people) tend to have a much higher average per capita income (Lux, Sweden, Norway, Switz, being examples) than larger ones (ex. US, India, China, Indo, Russia). I wonder if Norway or Sweden would be in such good economical, ecological, and social conditions if you stacked 300 million people to that number…