The Ukrainians are inflicting heavy losses on the Russian side (including Wagner mercenaries) without taking losses that are nearly as heavy. The Russian side are just throwing lots of soldiers and machinery into badly-placed offenses so they’re playing into the Ukrainians’ hands as the Ukrainians continue to degrade their army. The crucial difference is that the Ukrainians, alongside being much more motivated, are being much, much smarter in terms of logistics.

If you’re interested in this kind of thing I highly recommend following Phillips O’Brien on Twitter, who’s a professor of strategic studies at St Andrews university and has been calling things very well since the beginning of the war: https://twitter.com/PhillipsPOBrien