I've said it before, the Russians simply don't have the man power to maintain an occupation of major cities. They can't afford to keep a large enough force in them to maintain order, because they need to move on to their next objectives, leaving behind a token force which will inevitably get crushed.
The Russians can take villages, due to the fact that the Ukrainians don't even seem to be trying to contest small villages. The Ukrainians know their major advantage is the cities, the terrain, and the fact that Russian forces are having to stretch their troops thin across the occupied territories. The more the Russians occupy, the worse the situation becomes for them.
The fact that the Ukrainians are on the defensive, yet have enough forces to match the Russians 1:1 is a huge blessing for them. They can spare troops to harass enemy lines, or even attempt retakes of occupied territory where Russian troops are thinning out.
