I agree with Susano. Especially because the effect would be bigger for Germany and France which lost significant more people.
Of course the biggest effect are that we won't see such a drastic end to emigration of German, British and French citizens to around the world.We may see South Africa, Kenya and Rhodesia getting a significant larger White population, while in the German colonies we may see Namibia turn into a majority White state, while Tanzania turn into a German Kenya. While for the French Algeria are unlikely to become majority settlers, we may see the French settler become such a significant part of the population, that it stay integrated into France. For Italy even with it heavy loses demographic the war had little effect, but without WWI Italy may deal with Libyans a decade faster, rather than keep fighting insurgencies into the 30ties. Libya like Namibia are one of the few states which have a serious chance to turn majority European thanks to its low population. Of course the problem are that the colonies will turn more and more popular, and I doubt they will last to 2000, even if any Great Wars are avoided*.
Demographic European France and Britain are unlikely to be much different, France received heavy emigration to make up for the loses, while Britain didn't really lose that many. Germany are the one likely to be different, I once made some calculation where I compared their population growth with different (non-Anglo-Saxon) Germanic groups, and the most conservative one the Flemish growth rate would result in a population close to 100 millions, while the more radical ones the Danish and Dutch ones would result in a population between 110-130 million. Austria-Hungarys growth rate are more difficult to follow, but a rough calculation would end up with a population around the same as Germanys**.
*Not entire unlikely, France and Germany are likely to grow closer if the peace are kept for a generation more.
**While Austrian Germans are likely to have a lower growth rate than the more rural Hungarian, Romanian, non-Czech Slavic populations, we would likely see the German population precent (25%) stay stable or slightly increase thanks to large scale emigration to and assimilation in Vienna*** and a few other important German speaking cities. Czech are likely to decrease in precent of the Empires population but their population will still grow in absolut numbers. Italians are one of the few groups to decrease even in absolut numbers, when German and Slavs move into the Italian dominated towns of Küstenland and Dalmatia.
***Vienna even in a Austria-Hungary without reforms arwe likely to grow to around 5 millions, if the Empire are reformed it may grow even bigger. A good rule of hand are that in Europe a capital make up 10-20 precent of a countrys population (Germany excepted for obvious reasons)