I'm not sure if this will be correct it's been a while since I did science but the population size influences if the species will survive or go extinct.
According to what I've found on the internet this seemed the easiest explained there are more individuals trying to use the same quantity of resources. This can lead to competition for food, water, shelter, mates, light, and other resources needed for survival and reproduction.
Therefore it affects the well-being of people as they will begin to fight or bump into issues regarding the number of resources they have this could be caused by overpopulation.