That's because we aren't actually fighting WW2 or WW1 today, artillery have little effect on prepared defensive position(eg concret steel reinforced bunbker). Prepared Defensive position plays both side. But when you are an attacker, you are required to move out of your prepared position to attack (otherwise you will just be sitting in your line and they are sitting intheirs) and that's where the casualty number is from. You will be attacked when you are out in the open, being ambushed.
Artillery and Air power on your own side only serve to NEGATE the effect so you don't suffer more casualty than you have to, no war fought in 21st century where you can level a town and walk in unopposed. Because even if you do, the line behind, and around will still be able to attack you once you are out of your PDP. So Russia is ALWAYS going to suffer more casualty than Ukraine, just how many more is the question here, again, refer to the conventional estimation on previous post
The only way Ukraine is losing more men to Russia is if the Ukrainian is retarded. But seeing Russian still not manage to achieve 'their war aim almost 3 years now, even tho it has shifted 3 times and then get themselves invaded as well, that's unlikely this is the case