Why were Chinese city walls often built much thicker than European medieval walls?

Answer: Their rammed-earth cores needed a broad base to stand, so the walls were built as thick banks

Chinese builders lacked the know-how to build thinner walls — Chinese builders put up thin brick walls, tall pagodas and long timber-frame halls whenever those were wanted, so skill was not the limit. A rammed-earth rampart is thick because that is how earth stands and because a wide top was useful, not because anyone was unable to work out a thinner wall.

Chinese cities had far bigger populations to shelter inside — City size is not what sets wall thickness. Some medieval European cities were large too, and a bigger population calls for a longer wall around more ground, not a thicker one. Thickness follows from what the wall is made of and what has to move along its top.

Their rammed-earth cores needed a broad base to stand, so the walls were built as thick banks ✓ — Most Chinese city walls were built as a core of rammed earth, often faced with brick, and earth cannot stand as a thin free-standing wall: it holds its shape only as a broad bank with a wide base, so bases of 10 to 20 metres were normal. The top of such a bank also doubled as a roadway for troops and, later, cannon. Medieval European walls were typically stone or stone-and-mortar, which can hold a steep face only a few metres thick. Same purpose, different material, different shape.