No war before WWI ever had a large enough number of combatants or was deadly enough in general to make a real dent in the population.
I think that’s fairly inaccurate. Just to pick the first example that came to mind:
By all accounts, the population of Asia crashed during Chinggis Khan’s wars of conquest. China had the most to lose, so China lost the most—anywhere from 30 to 60 million. The Jin dynasty ruling northern China recorded 7.6 million households in the early thirteenth century. In 1234 the first census under the Mongols recorded 1.7 million households in the same area. In his biography of Chinggis Khan, John Man interprets these two data points as a population decline from 60 million to 10 million.
I think that’s fairly inaccurate. Just to pick the first example that came to mind:
Source: Twentieth Century Atlas—Historical Body Count (necrometrics.com)
I haven’t checked how much of the decline is due to battles, and how much to indirect causes such as disease or famine.
What about people just going somewhere else? I would think migrations would play a role here but not sure just how much to expect.