Honestly speaking, based on China's population (1.420 billion as of 2022) and project it onto the current geopolitical scenario facing the world right now, I have two schools of thoughts. They can be controversial, but reasonable.
School ot Thought 1 - If China's population could be smaller, that means every single person in China would have more resources to utilize and share with everyone. Natural yet precious resources like clean water, good food, fresh air and fertile land, etc.
To be honest, China is very blessed to have Tibet and the Himalayas shielding the Chinese heartland from the Indian subcontinent (i.e. India, Pakistan, Bangladesh, Nepal, Sri Lanka, Bhutan & Maldives). In case anyone here doesn't realize, the Indian subcontinent which only has 45.8% the land area of China is now supporting the life of 1.9 billion people - that is, 135% of the population of China (and still increasing at a considerable pace).
That means with global warming, worsening climate conditions and overpopulation right now and in the coming years and decades, things are going to get much worse and much more challenging in the Indian subcontinent. And guess what would happen in case people turns from fighting over oil and gas, to fighting over water and land? This is especially when we have two countries in the subcontinent which are both #1 - At each other's throats; and #2 - Nuclear-armed.
Recall that both countries almost went to full nuclear war in 2001-2002. A meterorite which exploded over the Mediterranean at that time could have triggered nuclear exchange between the two countries should it explode over those two countries instead.
Pardon me for saying this (especially for anyone in the SDF that comes from the subcontinent), but in my view, the Indian subcontinent is a powder keg waiting to blow, as long as no concrete measures are taken to resolve the issues above. Hence, the Tibetan and Himalayan mountains are perfect natural shields for China from whatever is going to happen south of that barrier in the foreseeable future - all while still maintaining sufficient amounts of natural resources for her own people. China must make sure that those mountain ranges be held and controlled at all costs.
School of Thought 2 - If China's population could be bigger (threoratically speaking, it already is), then the probability for China as a civilization state to survive in the event of a major catastrophe happening to China would become greater.
This is in light of how we have seen the US-led West are marching towards full-on fascist supremacist N4z1sm, and that they would never accept the fact that a non-white country would be surpassing them in nearly every metric. In case of a major confrontation between China and the US-led West (of which Taiwan would be the fuse in my opinion), they are most likely to just go with full scorched-earth against China and the rest of the world, rather than surrender and accept the new reality.
(Rest of the world here refers to countries that have large populations and thus, having greater potential and would be targetted as well, e.g. Brazil, Indonesia, Nigeria)
Remember that China does not yet possess a credible nuclear parity with the United States based on current estimates (and may not be for the good part of this decade unless China is somehow able to spam nuclear warheads like dumplings), of which I would like to keep the figures conservative to not appear overconfident. Say, one day when the global and regional geopolitical situations become so unfavorable for the US-led West that they would resort to nuclear weapons as to wiping out China's right to even exist, many Chinese population centers in the eastern half of the country would definitely become prime targets.
Hence, that's why I believe that for this school of thought, China needs to have more people - Such that in case nuclear armagedon did broke out and China gets severely nuked and losing tens or even hundreds of millions of people - The more people China has, the more people in China that would be able to survive the nuclear armageddon imposed by the US-led West in an all-out nuclear exchange.
Furthermore, the more people in China that would survive the nuclear armageddon, the more people China could utilize for rebuilding, restructuring, redevelop and hence, rise again from the ashes of nuclear fallout, which would thus better ensure the survival of the civilization state.
Plus, in my opinion, whoever can rise up the fastest in the post-nuclear-armageddon world - Would be able to reign supreme on this little blue dot in the vast expanse of the universe for the foreseeable future to cone.
China has faced almost near complete devestation of her civilization state on several occasions throughout the history of China, and managed to live and tell those stories to the future generations every single time. I believe China could achieve the same too, this time.
Whether these 2 schools of thought contradict with each other, or they can coexist with compromises made is subject to everyone's interpretation.
