Estimates are that the ratio between Russian and Ukrainian casualties has been around 2.5:1.
All one needs to know is that the Russians have the advantage in manpower, artillery and shells. According to the Ukrainians, the Russians have an advantage in shells of 10:1. It takes a special kind of Baghdad Bob to claim the side with a 10:1 shell deficit is inflicting the most casualties at a 2.5:1 ratio.
You are failing to take into account that it is the Russians who are assaulting across open ground against intrenched defensive positions. In the meantime the Ukrainians are remaining in the protective positions in the trenches. It is not just a question of artillery duels. There are plenty of videos that show the Russian casualties. A knowledge of trench warfare in WWI would support the conclusion of higher casualties among the attacking Russians.