Ukraine claims a lot of things, it's called war propaganda. We won't know what the actual losses are on each side until the war is over. It's certainly absurd to take Ukrainian numbers uncritically. In fact, it doesn't even match up with Ukraine having done multiple mobilizations now while Russia has only done one. If Russia was losing anywhere close to manpower Ukraine claims, then they would've had to do multiple mobilizations by now as well. Also, as many military experts have pointed out, this is primarily an artillery war and Russia has a huge artillery advantage over Ukraine. That's where vast majority of casualties comes from.