Ask Lemmygrad

Russia does not have heavy casualties compared to Ukraine. The initial assumption may only be true for equal opponents, which is not the case - Russia has gargantuan advantage in artillery, armored vehicles, total domination of the skies, powerful long-range strike capacity, and now even drones. It has more of everything, including total manpower, and its military industrial base is vastly more powerful than anything the Western empire can muster. If a boxer can only punch once and then gets punched 9 times - that's not "equal".

The other factor is the fact that we're talking about attrition warfare, where there's close to zero big arrow maneuvers, instead the Russian military is methodically grinding down the AFU until its combat capacity is negligible. That helps Russia keep its casualty numbers low, regardless of what Western propaganda press is trying to convince the public of. So it actually gets easier for Russia the longer it goes on - because at some point the AFU will not be able to hold the line, which is going to end up in the general collapse of the front line. If you destroy your opponent's ability faster than they destroy yours and you're more powerful to begin with - the collapse is a mathematical certainty.

Also - Russia does not want to occupy all of Ukraine, that goes directly against their economic interests and it is something they want to avoid if possible.