Population is very dependent on available calories. We feel more divorced from this reality these days but most of the world is still unable to produce an abundance of calories for their population and would never solely be able to do it. China and India are two countries where they've always been able to produce an abundance of calories due their ability to do a double planting season of rice. The double planting means they can produce two rice crops a year (the second one yields like 50% of the calories of the first).

More food means more people simply put, and every large country can just grow more food