Russia is about 146 million people and Ukraine about 47 million but that doesn't matter because armies are always much smaller, and you can't send draftees and policemen to fight in the frontlies. Armies fight, not the whole population. Also, Russian objective is not only Donbass region but destroying Ukraine as a nation and putting in Kyiv Russian marionette regime. This Kursk incursion proved that Russians are already stretched thin with vast borders with no defense, because they had no even two brigades to spare for the defense of Kursk oblast, that they are ineffective in strategic and operational level because they allowed Donbass offensive to degrade to positional trench warfare, and had shown very little of modern (WWII era and newer) warfare. Meanwhile Ukrainians will use river Seym as natural defensive line if the situation worsens. Also, Russians progress in Donbass region which proclaimed secession on Ukraine in favor of Russia where population is friendly to them while Ukraine in Kursk region does not and look at the gains of both sides since August 6th.
lol, Ukraine had a population of around 40 million BEFORE the war, now there are around 12-15 million people who have fleed the country or are missing and displaced + all the casualties of the war so far.
There might be 25-28 million actual people in Ukraine as of now, 50% of which are women, and from the other 50% which is around 12-14 million a huge chunk of them are bellow 18 or over 55-60
Realistically Ukraine has maybe 3-4 million men of fighting age
Russia's population was 143 million not 146 million before the war with Ukraine, lets count around 250-300.000 casualties so far (im taking US numbers don't give me those BS Ukranian propaganda numbers lol), so we are left with 142.7 million people.
Discounting women and people bellow 18 or over 55-60 we are left with
around 25-28 million men of fighting age
Summary
Ukraine 3-4 million men of fighting age
Russia 25-28 million men of fighting age
Its not even close, if Ukraine will be stubborn enough and grind this war long enough, they will pretty much be doomed, and their future also