What countries stayed, respectively, the first and second most heavily populated throughout the 20th century?

China and India were and remained #1 and #2 in population throughout the 20th century. In 1900, China had a population of 400 million people. By 2000, China's population had reached 1.3 billion. India's population in 1900 was around 250 million, and by 2000 had grown to 1.05 billion. For comparison sake, the United States' population in 1900 was 75 million and in 2000 the US population stood at 282 million. The population in India and China not only started the century higher than other nations, but grew at a faster rate. (Population figures can be found at Our World in Data.)