Disproportionate losses of Muscovites are believable for two reasons:

• Defenders are favored over attackers, and right now Ukraine is fighting defensively, not trying to take ground like the Muscovites
• Moscow’s complete disregard for its soldiers is evident every day seeing it march lightly armed men on foot straight into heavily fortified Ukrainian positions.

I’m sure there is some error and embellishment on both sides for their assessment of their and their opponents losses, but huge Muscovite losses are completely expected.