And like the article suggests it can be deliberate too. Am extremely skeptical of population figures in some parts of former Soviet Union. The official demographic loss figures in WW2 had tripled since 1945 but post-war census figures were never revised. That could easily account for the "demographic collapse" of 1990s.
Edit: changed world-wife (which sounds interesting demographically) to world-wide
Large cities are inherently inimical to living in large families. And yes, it was apparently the case even in the Roman Empire.