Why are Chinese city walls 5-10x thicker than European ones?

Answer: Different materials needed different geometry

Different materials needed different geometry ✓ — Correct! Major Chinese walls used a rammed-earth (夯土) core faced with brick, and rammed earth needs mass to stay stable — typical thickness 10-20 m. European medieval walls were solid stone or stone-and-mortar, which holds itself up vertically — typical thickness ~2 m. Same job, different materials, different geometry.

Chinese cities had bigger populations to protect — Wrong. Many medieval European cities had populations comparable to Chinese ones. The thickness difference comes from material physics: rammed earth needs mass to be stable; cut stone doesn't.

Chinese builders had no math for thin walls — Wrong. Chinese architecture had advanced load-bearing math from the Han dynasty onward. Walls were thick because the rammed-earth core needs mass for stability, not because of design limitations.