>>214657160
My guess is that because living as a rice farmer was relatively low in demand (hence why so many chinese) and china is chill and away from invading races other than the northern ones, they basically relaxed and began picking the smallest and most youthful wives, which in turn made rice farming asians become neotenous and small in general.
That in addition to the lack of extreme war,
Other continents had constant war (europe) or climatic hazard to make up for the easeness brought by farming.
And had much more food resources while rice farmers basically adapted to live for long and quietly, simply discarding muscle and agression genes and favouring simple carb metabolization for long term use and famine.
Very abundant places with constant wars favour those rhat exploit those resources the best
Think how during the age of dinosaurs animals were gigantic, that was because so much food was avaliable to feed them