The size of the population, as well as the rate at which it grows or shrinks, is not really a huge factor for any wide scale political trends in the current stage of history. As Dirt Owl said the main reason you'd be concerned is for racist and misogynistic reasons. Even the notion that more workers = more cheap labor isn't even that relevant, because the capitalist class doesn't need to worry about replacing the productive labor in the imperial core, they only need to make sure a solid consumer base exists to keep capital flow going. If not enough people are having babies they can just tweak wages, interest rates, and other parameters to keep the desired level of consumption. Meanwhile, in the periphery and semiperiphery, there is constant population growth from rural countries where families need to have more children to assist in farming, and many of those children grow up to move to industrialized urban centers (often outside of their own countries, see how many migrants from Bangladesh go work abroad for example). This population growth and migration is beneficial for capital, but it also creates problems for capital in the long term because it creates something like an apartheid system for migrant workers; the Marxist discourse on these kinds of dynamics is honestly beyond me but the long and short of it is, the thing you gotta focus on is the movement of capital because capital determines how labor will move in turn, so any problems that porky tells you are happening because of population growth are downstream from the movement of capital.