That then depends on your definition of success. Which I still would want you to provide.
Plus, I think terms of "success" with respect to a /nation/ is slightly skewed when compared to the "success" of genetics or species. I don't think that is an accurate cross-comparison.
I don't really need a definition of success, but I'd say a society that has a high chance of survival, has minimal suffering, and creates beauty.
Given all those things being equal, certainly a higher population would be more successful. However fast population growth is likely to work against most of those goals.
Plus, I think terms of "success" with respect to a /nation/ is slightly skewed when compared to the "success" of genetics or species. I don't think that is an accurate cross-comparison.