In addition to all the other valid arguments I want to also mention the rotation principle of the ukrainians. They deploy for six months to the Frontline and then rotate between the dugouts and a safehouse for two weeks at a time. So their soldiers have time to relax and eat good food even while deployed which keeps morale high.

Russia used to just keep their common troops on the frontline until they were exhausted. If I recall correctly they changed this in the last months, but they most likely lost almost all of their pre war trained troops.