Time for some more population estimates right before the Central European War broke out. I previously estimated Germany and Italy's populations to be 69,0 million and 40,1 million in 1915, using estimated growth rates of 1,228% per year for Germany and 0.814% for Italy. If those same rates were used for the 4 years from 1915 to 1919 then the following populations could be roughly estimated.
Germany: 69 021 138*(1,01228)^4 = 73 364 404 people.
Italy: 40 087 502*(1,00814)^4 = 41 408 774 people.
France had a TTL population of 46 810 371 in 1910. OTL from 1910-1913 they grew from 41.35 million to 41.62 million by 270 000 or 0,217% per year. Without adjusting for TTL higher birth rates and better economic development, it would give a 1919 population of 46 810 371*(1,00217)^9 = 47 732 553. OTL from 1901 to 1911 France grew from 38 962 000 to 39 605 000 by 0,16% per year or (1,0016)^10 = 1,61% in total versus TTL's growth of 3,8% from 1900-1910 which is (3,8/1,61 = 2,36 times higher). This would give a growth rate from 1910-1919 of 0,217*2,36 = 0,512% per year which equals a 1919 population of 46 810 371*(1,00512)^9 = 49 012 100.53.
Austria had a population of 51 005 617 in 1910 TTL and had a growth rate of 10% over the decade equal to 0,957% per year. We know that Austria Hungary is more prosperous than OTL and the 1910s have been a better decade for living standards (reducing things like emigration) and likely boosting population growth a bit (say by 10%) which then equals 1,053% per year and a TTL 1919 population of 51 005 617*(1,01053)^9 = 56 048 101 people in 1919.
The Iron Triangle's two main powers have a total population of 56,0 million + 49,0 million = 105-105,1 million, and the Central Powers have 73,3 million + 41,4 million = 114,7-114,8 million. Belgium and Denmark do go some way to correcting this and likely give the Iron Triangle a very slight population edge.