Population grows exponentially. The higher your initial population is, the more people you add to your last year's population. 1% of 100 million is one million, but 1% of 1 billion is 10 million. So, two populations with the same growth rate having different initial populations will grow very differently in absolute numbers.
As for being pure bred, first of all, in terms of evolution, that's a negative thing. We all know how populations with genetic variation have a far higher chance of survival. This is even encoded in our evolutionary psychology through choosing mates that have different MHC complexes than those of ours. Secondly, refer to my earlier point. A larger population has a higher chance of staying pure than a small nation. Again, not an achievement. Just basic mathematical facts and biology.
Thirdly, China was an underdeveloped poor nation before you opened your doors to the West in early 80s. You didn't prosper because you had famines. If it weren't for the greed and racism of the US as a Euro-centric power, China would've never raised to the position it has today in science and technology.
They thought they could enslave you and use you as a labor camp for pennies on the dollar because you were not intelligent enough to start building your own things after they moved their production lines to your country. It was a blunder by the US, caused by capitalist greed and white supremacy, thinking that an Asian country was not capable of being innovative like them. They helped you become a powerhouse with their own stupidity, not because you endured famines.