And a recent report identified 60.000 Russian soldiers that had been killed. If the number of identifiable bodies alone is that high, the Ukrainian numbers are most likely credible.
Remember as well that modern small arms rounds are built to inflict serious injury if they don't kill you outright - it takes more personnel to evac and care for a wounded soldier (unless you're doing what Russia is ALLEGEDLY doing and just killing soldiers too injured to fight).
UK intelligence has been more conservative in their figures but not by a significant amount. They often report a milestone within days or a fortnight of Ukraine.