India has been the second most populous country in the world and has been a democracy since independence.

Autocracy can happen anywhere and to any society regardless of population.

I have my theories around this - but also worth remembering that early governments didn't really understand modern economics or a knowledge economy - the most recentl iteration of Chinese government is only a few decades old and even they have been regressing in terms of institutions with nothing positive to show for it.

The Deng Xiaoping -> Hu Jintao period had a lot more achievements then the period prior or after and at that time the CCP was relatively more open and more rules-based rather than personality based.

In the long arc - I think successful governments needs meritocracy, which has both a technocratic and a democratic component. The technocratic component is needed to make sure things are executed well and complex problems are well understood - and the democratic component is needed so that the technocrats don't become divorced from reality and turn into a kleptocratic elite that starts acting against the benefit of the majority.

This doesn't necessarily mean a 1-man-1-vote standard democratic system - but you do need a government that is responsive to the public in some form. In Singapore for e.g. you can't really call it a democracy but the state is quite responsive to public issues. On the other hand you can have a kind of democracy where you don't really have a lot of competence which will lead to people choosing idiotic self destructive policies. Most successful advancing countries have found some balance between these two.