Wikipedia has estimations made by different entities, but they vary wildly as others report only offically or otherwise confirmed cases (which of course differs from estimations by quite a lot) and others have different ways to estimate things.

But in general, the Ukrainian numbers we have here almost daily aren’t that far off from other estimations and they at least have a front row seat to give rough numbers. Obviously they have incentive to give a bit optimistic estimations from their point of view, but they also have incentive to stay as close to the truth as possible for PR reaons among other things.

And finally, Ukraine has a totally different doctrine on their military. They value their people and want as many as possible to return and rebuild their country. Russia doesn’t seem to care about losses at all and they just care about the results no matter how many dies while reaching them. And that’s been Russias tactic forever, inherited from Soviet Union and tzars before that.