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.