If population doubles, then demand doubles, and so will the monopoly profits. The more people available to benefit from an idea, the more likely it is to be worth doing. This principle holds up empirically. In general, more population means more demand for everything. This means that more products will be available, and more available ones will be cheap.
More population means that ever more eccentric tastes can be profitable to satisfy.
Since the number of available quality levels is just a special case of variety, the same principle holds. Of course, it is better for you to have more people who share your tastes , rather than just a random sampling of people. More people who share your tastes means more demand for your favorite varieties and quality levels - so they get cheaper and more abundant. Since more quality would cost more, the low quality of goods in poor countries is a blessing in disguise. Including the number of available quality varieties.