Of course there are population outliers, but even after I took out all the countries with populations of over 100 million (there are 12) no relationship appeared. In any event, from a policy-making standpoint, it's not clear what this would've meant anyway. (Quick! Join together with bigger countries so we'll get higher GDP!)
There has been work done on population growth rate (below, source), showing a negative relationship; interesting but not surprising, likely relating to demographic transition. Additionally, one of the effects of being in a country is that of the same currency across the population, so a separate question would be whether monetary union regions grow faster than non-union regions, but there are far fewer data points there.