The rich get richer and the poor have children. Different population growth rates are causing dramatic changes in the compositon of the human family and these are closely linked to economic outcomes and all sorts of other issues. Classical economic theory makes growth a direct function of labour force. In simple form this leaves out the big investment in human capital needed in a modern workforce. Moreover, slower demographic growth takes pressure off land use and construction, freeing resources for other uses.
Slower growth makes it much easier to manage city planning and infrastructure.
Resources are freed from building more houses and so can create other capital projects, giving productivity increases that will increase per capita income. The history of land use regulation provides an important analogy. As density of land uses increased, land uses began to impinge more on each other. As externalities became too large to ignore, land use regulations were enacted. In the same way, as human populations have grown, your decisions to have children come to have more effect on the life outcomes of my child.