Well, population also matters in todays modern world to be honest, especially when you have the appropriate working system and policy for your country set. Since it also gives you scale. Even if China is half as productive as Japan for example , China will still have a bigger economy than the US for example , this is the advantage of scale and population. Do you think if China had the same population as South Korea or Japan or U.K, Germany etc you will be as powerful as you are today ? No, China will have an even smaller economy than those countries to be honest . The only advantage you have over them is your scale and size/population. Reason you stand a chance to even challenge the US. Without that population size, China will just be a medium size power at best like South Korea. In the same respect, if Japan had your size and population, they will have been a super power as well long ago. In fact that scale and size coupled with their level of Industrialization/technology , they will have dwarfed the US in basically every scale imaginable long ago, and by default be a superpower setting/dictating rules by themselves as well just like the US has been doing. One of the main advantage the US had over them was the size/scale/resources and population of the US which dwarfed Japan by far in every of those field, else I don't even think it would have been possible to challenge and defeat Japan even during WWII to be honest. Even with all those advantages, it was a tough battle for the US to defeat them.