lol, looking at people talking about manpower just by looking at population.
In the military. We don't talk about manpower this way; the correct term is "Force Generation" and "Force Regeneration" because even if you have a 10 million force (which both sides actually can raise) that does not translate to you can put all 10 million soldiers on the ground. The soldier you can put on the ground depends on a multitude of factors. Including available manpower, support size, means of control (ie do you have enough people to lead them), and needs (ie, you have other needs for troop, rear echelon stuff, base defence, border defence, and so on) that make up the pie on how much troop you can put into a theater TO&E. And this magic number for both side now is roughly 750,000 for Russian and 400,000 for Ukraine. Judging from the number of divisions and brigades in the theater
Now, let's talk about Force Regeneration, in the shortest and briefest form, itis basically an equation of the following
Battlefield Loses (Desertion + Casualty + Regular Rotation) < New Troop
The number above means Russia needs to maintain a level of 750,000 troops in the theater to have a positive Force Regeneration, while the Ukrainians need to maintain it at 400,000
Both sides cannot match that because of several different factors. For Russia, casualties are simply too high to match that number; the Russian casualty is estimated to be 1.8-2.1% of the Ukrainian Casualty rate. The problem is, Russian forces is a lot bigger, which means in absolute numbers, you are talking about 150,000-200,000 new influx to maintain that net loss. The issue is, Russia is not recruiting enough people. Average Troop rotation is about 110,000-120,000 with conscripts.
On the Ukrainian side, first of all, the 400,000 troops they can put in battle are not enough to defend all these territories; they need 550,000 troops, which they also can barely match. Because of the training rate and recuperation rate. Contrary to what many people here believe, desertion is not a net loss of manpower. Well, if they get caught and are executed, then it is, but once a soldier deserts their post, either they will go back willingly, or get caught and sent back; it's not like they deserted and were allowed to leave the country or deserted toward the Russians (Not massively happening while some individual case exist) or the Ukrainian will just let it go and not bother to look for him. If a soldier deserted the military, they are still in the country.
Now, the problem is, yes, Russians have an advantage in the absolute number (750,000 vs 400,000), but that is not much, which is the reason why we are seeing the Russian progress slowly, because they can only fight with local superiority. The problem is, as Ukraine loses territories, Troop Concentration increases as you need fewer troops to defend its land (Simply, you need fewer troops to defend if you have less land). So, there will be a time this advantage stops, especially since the Russians never had the conventional superiority (Attacker needs to be 3 times the size of the defender) this is the reason why Russia take longer to take towns and stablise their line, because their line is increasing, which mean you need more manpower to cover that (as in regenerating force), they don't have that either.
If this war is going on forever, or in a super long run, beyond say 20-30 years, yes, that advantage of raw manpower and raw recovery power is going to play a vital role in this war, But in a short term like 3 to 5 years, that advantage does not exist because both side does not have that much diffent in term of Troop Concentration.