The "power" of a country, however you define it, lags the population growth. China was the biggest country for decades, its power only really started exploding some 10 or 15 years ago.
That's an incredibly short sighted way of looking at countries. The US was never the country with highest population growth, and right now is also heading into a demographic collapse.
Peter Zeihan "Don't be Surprised by China's Collapse":
https://www.google.com/url?sa=t&source=web&rct=j&opi=8997844...