As already stated, casualty numbers - by both sides - should be taken with a grain of salt.
But whatever they actually are, Russian casualties are probably higher - a LOT higher - than the Russians were projecting on the day before they invaded.
I think a good part of the reason is that as a former Soviet state, the Ukrainians were well-versed in Soviet (e.g, Russian) military doctrine, tactics, and hardware, and were able to use their knowledge to good effect. Especially since the Russians were telegraphing their intentions for months.
I wonder if anyone outside Russia's military has a good handle on how Putin's stocks of missiles, ammo, combat vehicles, and aircraft (fixed wing and helos), and spare parts are doing, given combat losses and high tempo operations. By some counts, Putin's used over 1300 cruise and ballistic missiles already, including at least some new and expensive hypersonics.