Russians outnumber Ukraine so they got 40 million more Russkies (just throwing a random number out here) to throw at Ukraine. And historically that's how Russians have turned the tide in a war. They throw away men like you and me throwing away a candy wrapper cause they got plenty of men to throw away.
Ukraine killing more Russians than Russians have killed Ukraine (which i don't know if thats true or not) would mean nothing cause the Russian army still would greatly outnumber the Ukraine army.
Its like if you had 100 guys but I had 1000 guys. I kill 50 of your guys, and you kill 150 of my guys. Yeah you got more of my men than I got of yours but guess what?
The 850 of my men that's left is about to give your 50 remaining men that work