First of all, no one said 3 billion people. India's population will not reach near that. Secondly the more people you have, the more workers and teachers, and thus the bigger your bank. It's easy to maintain a strong education system for a large population. Thirdly no one said education wasn't important. But a highly educated population of only 10 million or even 100 million will not be able to be a significant world power able to stand up to the US.
. Is Canada a world power? You need education AND population.
China's net calorie intake has been
It is doing better than ever. For the 5,000 years of Chinese history, calorie intake has never been higher. If China has survived for 5,000 years up to now with far lower calorie intake, it will continue to survive. Furthermore
is accelerating. China can easily maintain a population higher than India with a high calorie intake suitable for rich people.
No one's is saying you shouldn't become more productive. You can become more productive and grow your population at the same time. That way, you get the multiplier effect of more workers who are also more productive. That allows you to cut working hours and actually improve the quality of life, while still having a larger economy than with a smaller population.
You forget that with a larger population, you also have a larger workforce to support new industries. This is how countries build powerful economies. More people => more industries. Further with more workers and industries, you have higher tax revenue to support your people. As far as food, I already addressed that above.
Of course productivity and GDP per capita should grow. But so should population. For a civilizational state like China that seeks to maintain equality despite many opponents, it needs to maintain its relative population level compared to hostile great powers.