Raising or lowering the population does not change the maximum GDP, or the costs of policies that should have a per-person cost.
This seems like it wouldn’t matter immediately, since the percentages would be the same, but in theory one should be able to combat debt by increasing the population in a surplus-generating government, thereby increasing the surplus, or vice versa.
Furthermore, population changes do not change the youth demographic, when they come from birth policies and not from immigration. They should have an effect on “youth” and “retired” which should last for about 25 years.