"why isn't China more successful?"

Compared to what? to a contra-factual or to a real country?

Because 500 million people out of extreme poverty sounds pretty successfully to me. Specially if we compare it to any other country following more "open" strategies.

Compared to South Korea, Taiwan, Singapore or Hong Kong, which started just as poor as mainland China 60 years ago but leapt way ahead. And if the response is that China faced more difficulty due to the larger population, then surely the best approach would have been to split it up into multiple smaller countries?

I don't even know how to address such a naive argument. Split the country into pieces? It doesn't take a political scientist to see how impractical this is. I didn't realize economic growth became a goal of such paramount importance to the government, that it should consider willingly breaking itself apart to achieve it.

Just one counterargument I can see - China floundered economically under Mao, and only in the late 70s did real economic reform under Deng Xiaoping happen. An effective timeframe of <40 years, not 60-70. I'm sure other Asian countries experienced poor leadership at times, but to this degree? To stagnate as heavily as China did under Mao?

Lastly, size definitely matters, in terms of both geography and population. Even today a huge number of the population is dispersed across the rural regions. How can economic prosperity reach them? Much easier in other Asian countries for growth to accumulate in centralized economic centers and start improving the welfares of the population at large.

Well, the divergence of China and Taiwan is a natural comparison.