Comment of the Day: Graydon: Mobilization for Total War and Democratic Order: "Great War mobilization levels are fundamentally voluntary...
...(Note that the US did not meaningfully participate in the Great War and if want a reason the US is different about almost everything compared to the rest of the North Atlantic, there you go.) You can't possibly impose them, there aren't enough people to do the imposing. Hitler's War more complicated but not politically different.
If you look at personnel counts you're going to get the wrong answers; look at percentage of the population. Look at whether or not there was (an intent for) a decisive result. (Colonial wars are not (directly) about how industrial states relate to their own populace and political legitimacy.)
Yes, revolutionary France invented mass armies; they were not decisive. (Scaring your neighbours is not decisive.) Yes, there are existential wars all through history. The difference is that you get a situation where you expect that your survival requires mass armies and will involve an existential conflict. (In part because it's possible due to logistical improvements that go along with the industry that can make drawn brass cartridges.)
The change with metal cartridges is threefold: you need to go to dispersed infantry tactics, which plays hob with your previous mechanisms of articulation and control so you have to invent a new army; your effectiveness is the number of accurately-shot rifles you can get into the conflicted volume; there is no counter (pre-1915!) to rifle regiments except more rifle regiments. Suddenly the feedback mechanism is open-ended; you have the industry to feed and supply all the troops you can raise. Great War mobilization rates hit 15% of the total population in industrialized primary participants. (The French hit 21%.)
You absolutely must have the voluntary participation of your population to do that; you can look at the Great War and note that only the British Empire survived of the participating empires because trying to keep that participation rate shredded the pre-war social order. (the British one, too, but not quite to the point of changing the formal basis for the legitimacy of government.)