Having fewer people is only good in the scenario where you cannot organize people and intermediate disputes effectively. Having more people leads to things like electricity, transportation technology, computers, modern medicine, and space travel. It also results in weapons development (see first sentence.)
China has GDP per capita below Costa Rica[0], and yet can determine actions and statements made by enormous US organizations like NBA [1], and in knock-on effects, statements made by Disney-owned ESPN about statements made and subsequently disowned by NBA [2]. Multi-national organizations don't think twice about Costa Rica.
Matthew Yglesias makes the case in a recent book that the US should embrace all the ways that the country is able to continue growing while China's, and many other countries', populations stagnate and decline.[3]