You argue that the major defining factor of population size is food limits. Australia (to give an example), currently has a birthrate less than 2 births per woman. We have an overall annual immigration, so our population is growing, but if we had no immigration, our population would be decreasing. Australia is a fairly affluent country: plenty of food, people are educated, well supported with social services, and generally feel secure. They don't need the added security of a large family (I don't claim that this is causal, but believe it may have some impact).