WW1 France had over 300k boys turning 18 in each year of the war. Health conditions at the time were worse so some had exemptions, but socially it was not a problem to draft most of them and send them to the trenches. Ukraine doesn’t mobilize 18-year-olds and in any case they are a fraction of the number that France (and the other major powers of WW1) had available back then.

Also, I think the KIAs estimates from the leaks were definitely underestimated for both sides.