system
Given the following post, respond as if you're commenting on Reddit:
human
Many believe a growing population is required to sustain economic growth yet a smaller population means everyone gets a bigger slice of the pie. Which is better?
gpt
TLDR: It's generally accepted that population growth is better.

This is a topic which falls under the purview of endogenous growth theory, which is based around a basic model called the Solow-Swan Model. The model is really simple, it just assumes that the output of an economy is some function of capital assets and labour, modified by Total Factor Productivity (TFP) which can very loosely be understood as technology. Capital depreciates over time but increases with investments, labour increases with population growth. You can add any number of modifications for human capital, limited resources, etc but this is the basic model. In university, you'd be taught these as a whole bunch of equations, but let's ignore all of those and focus on the intuition.

The main purpose of the the economy is to maximise the output per person (i.e. GDP per capita). In practice, this means there are three ways to increase the total output of the whole economy permanently. The first is to increase savings (and hence investment) rates, which increase the stable state of capital. But saving more to invest later means consuming less now, so in effect, there is a maximum level of savings which maximises output in every period. The second is to increase population growth rates, but the more labour there is, the lower the GDP per capita is, so there's no net effect. The third is to increase TFP or technology growth - which by definition allows us to use labour and capital more efficiently to make outputs.

Increasing technology growth isn't that simple. By definition, TFP cannot be something we can directly invest in (otherwise it would form part of the capital bucket). However, labour and capital clearly have some kind of dynamic with technology. This is called endogenous growth - growth which creates its own growth. We have both capital-led theories and labour-led theories for this. 

One of the capital-led theories is called learning-by-doing, and it simply argues that having more capital gives engineers/scientists more things to research and improve, so economies should aim to be capital-intensive as that's a virtuous cycle. On the labour-led side we have something called standing-on-shoulders, which argues that having a larger population means we can have more scientists. Since even a super mega breakthrough can be discovered by a single person, having more scientists scales exponentially to it's effect on the population. Obviously each of these theories have much larger and more complex mathematical derivations and policy implications, but this is a crude summary.

There's lots and lots of different endogenous growth theories. Some like Acemoglu's (2010) Directed Technical Change are really very complex; while others don't deal with the labour/capital dynamic but maybe something like finite resource problems, natural geography, and even presence/absence of democracy. The field itself is relatively new, and we don't have enough empircal data to really test it, so it's one of the more exciting fields of economics where there's no real consensus on how each of the effects come together. What's clear is that real life is a combination of multiple often conflicting endogenous growth theories, and no one effect alone is enough.