I already did a thread about this months ago, but i felt like i could have phrasing it better.
Its common sense that the more people there are in an economy the more it will flourish, as there are more people paying taxes, more people opening their businesses, more people in the workforce etc…
So i think that it is a bit of an oversight the fact that the game does not reflect that and instead treats Population growth and immigration as a looming net negative for the country.
And while is true that the more the population the more the recipients of welfare, the negatives are easily offsetted by the GDP growth and/or a smaller welfare state.
TLDR; More population should equal more GDP. It is factually incorrect that population growth is just a negative but that is what the game portrays right now.