you dont need a trait for a large, or rapidly reproducing population.
but also keep in mind its not that simple... more people means more soldiers, crewmen and shipbuilders, etc. sure. but it also means you have to train, feed, and manage more people, possibly pay them as well. large nations are more creaky, they have more corruption and bureaucratic red tape, and the overall quality of your personnel would be lower. additionally a larger population means more dissenters, which in turn means they are more likely to form cohesive freedomfighter/terrorist groups, harassing your social order. social Flaws, like Public Outcry, would hit you alot harder.
take for example, modern day China. last time i looked it up, they had more people in their military than the USA has People
on its west cost. however, large sections of that military are pooly trained, or using WW1 ear weapons, or both. in recent years, china has been downscaling quite abit, in order to establish standards of training and equipment.
this is mentioned on the nation-sheet under number of planets,
(Somewhere between 1-50 is okay. Any nation over 25 is... creaky, and suffers regional problems, which must be included in the Weaknesses (terrorist groups, revolts, slow reaction to threats, corruption, etc...)