Here's a figure of population totals for the two countries. Back in 1970, China's population was 818 million, while India's was 553 million. By 2000, the gap had narrowed to "only" 150 million or so, with China's population of 1,262 million and India's of 1,053 million. By 2016, the gap was down to about 60 million, with 1,378 million in China and 1,324 million in India.
Population growth rates have slowed substantially in both China and India, but India's population growth rate has been higher since the mid-1970s (after China adopted an extremely aggressive family planning program in the early 1970s). The population growth rates have converged somewhat, but India's annual rate of population growth remains about 0.5% per year higher, so it closes the population gap by about 6-7 million people each year. In about a decade, India's total population will overtake that of China.
The total population figures mask another difference: China is aging much more rapidly than India. For example, here's a figure showing the "age dependency" ratio, which is calculated as the number of elderly divided by the working population. Thus, in China the age-dependency ratio has already reached 14% and is rising sharply, while in India it is at 8% and rising more slowly.
As the saying goes, China seems likely to face the problem of getting old without first having become rich. For India, the key question is how to help its large and growing working-age population be economically productive, before India's age-dependency ratio rises, too.