The two most populous countries are China and India, respectively. Both form the world's largest manufacturing hub, with the lowest wage cost to capital investment. Incidentally, China's population is on the decline, however I am not entirely sure what you mean with
I can't be sure as to whether or not you're advocating eugenics. Which, if it be the case, I must strongly disagree. Eugenics is a poisoned chalice. Through its history, men of science have always thought of it in ambitious terms; whether eliminating undesired types, or increasing desired types, they have their reasons. But they all make the same mistake: They imagine that a reform inaugurated by men of science would be administered as men of science would wish, by men similar in outlook to those who have advocated it. These are, of course, delusions; a reform, once achieved, is handed over to Power. So, if eugenics reached the point where it could increase desired types, it would not be the types desired by present-day eugenists that would be increased, but rather the type desired by the average official. Prime Ministers, Bishops, and others whom the State considers desirable might become the fathers of half the next generation.
If we knew enough about heredity to determine, within limits, what sort of population we would have, the matter would of course be in the hands of State officials, presumably elderly medical men. Whether they would really be preferable to Nature I do not believe. I suspect that they would breed a subservient population, convenient to rulers but incapable of initiative.