He presented the first one in a lecture linked below at the Global Priorities Institute.
A few reasons we might at least want to consider believing more people = better economy
Large increase in GDP per capita in the last few centuries was associated with a large increase in the global population. Obviously some of that increase in population is due to the economic growth itself but the connection could run both ways
The per capita cost of R&D is smaller in a larger population, but the benefits are the same, since unlike in the production of goods, ideas can be shared by the whole population.
More speculatively and qualitatively, a society which is growing in population is younger and potentially more dynamic and future-oriented than a society which is declining and aging
A reason for skepticism is that while population size is plausibly a major factor in long-run growth, it doesn't seem to have been the main factor in recent US growth. From paper 1 above,
Section 3 conducts a growth accounting exercise for the United States to make a key point: despite the fact that semi-endogenous growth theory implies that the entirety of long-run growth is ultimately due to population growth, this is far from true historically, say for the past 75 years. Instead, population growth contributes only around 20 percent of U.S. economic growth since 1950. Rising educational attainment, declining misallocation, and rising (global) research intensity account for more than 80 percent of growth.