I ended up on this Wikipedia article and was pretty taken aback. #1 is WW2, #2, 3, 6, 7, and 8 are all Chinese civil wars, and #9 is WW1. The war of three kingdoms is said to have reduced the population of China to 35% of what it was during the Han dynasty. The An Lushan rebellion (by high estimates) reduced the WORLD population by 1/6. How? Why? What was so different in China than everywhere else?
A lot of caution needs to be used with assessing Chinese casualty claims and population estimates, especially in premodern times. Usually these counts were of taxable households rather than direct population counts, and were affected by changes in the effectiveness and extent of central authority. The An Lushan estimates have been made famous (or notorious) thanks to Steven Pinker, but actual experts on Chinese history don't think the war was remotely as devastating as that claim is.
A good example is in the radio discussion of the An Lushan Rebellion on "In Our Time" on BBC Radio Four (it's available through the BBC website). The guests- Frances Wood (Lead Curator of Chinese at the British Library),Naomi Standen (Professor of Medieval History at the University of Birmingham) and Hilde de Weerdt (Fellow and Lecturer in Chinese History at Pembroke University) discuss this point.