During WW2 the high soviet casualties were a result of the Axis killing POWs and killing civilians en masse on one side, on the other side was the situation from 1942 onwards. The Red Army was pushing the Axis back, which meant the latter fought from defensive positions while the former did not. Attacking always results in higher casualties in a fight of roughly equal forces.

People love to forget that Germany went into that war with the intended goal of reducing the soviet population by about 30 million. Casualties of the USSR, both civilian and army, ended up between 23 million(lowest estimate) and 40 million(highest estimate), thus the Nazi fucks achieved their blood toll.

Meanwhile the western allies were chummy with the Nazis, with massacres of their populations being rare occurances.

One side received the full brunt of the Axis warmachine and savage ideology, the other was toched with kiddy gloves.