First of all, Russia is NOT Soviet Union. Ukraine contribute as much as Russia does in WW2 as part of Soviet Union.
Secondly, 800k is NOT really a big number, consider you will have a lot of troops sitting idle defending fronts that may or may not have got hot. There are 4 to 6 Brigade sitting in Kyiv at the moment with no war going on. There are 1.3 million troop in Russia, that does not mean Putin can pull them all from defending Russia to fight in Ukraine, the same with Ukrainian military.
Finally, Russia ALREADY lost this war in strategic term, their goal is to demilitarize and denazify Ukraine and stop NATO from expanding. Instead, NATO have gained 2 new members, possibly another 2 (Georgia and Kosovo) And instead of Demilitarize Ukraine, Ukraine has just overtaken Israel as the world most Militarised country on earth. And I don't know how you can denazify Ukraine but seeing Russian won't be able to take the entire Ukraine, I don't think that's possible to denazifying them.
What Russia want now is a tactical victory so at least they said they have achieved something, but that tactical victory means Russia have to capture the entire Donbas (what they said), right now, even before this counter offensive, their force aren't going anywhere, they have been trying to take Siversky (9 miles from Lysychansk) and Bakhmut, and they still fighting on the outskirt even though Lysychansk felt in July. Now with this counter offensive and they lost Kupiansk and Izyum? The Entire Donbas is basically at the mercy of Ukrainian, because you don't have supply going in and out of the area now Kupiansk and Izyum is gone. Russia literally need to shift their entire focus on supporting the Operation in Ukraine from Belgorod to Rostov as you can no longer support the war from Belgorod as the rail line between it and Isyum was cut. And I don't think you need to serve in the military to know you cannot switch logistic base in just a month, and then winter comes, what supply can Russia use to fight during Winter??