Those percentages are only among republics, not total population. Here's an actual study
The first notable trend is that the majority of soldiers who have been killed in Ukraine are Slavs - and primarily ethnic Russians. The number of ethnic Russians in casualty lists more or less matches the overall proportion of ethnic Russians as opposed to other national groups in the male population of the Russian Federation.
As the war continues, the percentage of Slav names among the dead is increasing. In the spring 2022, it was at 75%, and a year later, it rose to 85%. Researchers attribute this to mobilization and the enlistment of prisoners to the front lines.
The second trend Bessudnov has been able to shed more light on is that while Buryats and Tuvans are clearly dying in disproportionate numbers compared to their share of the overall population, the numbers of young men from the two regions being killed as a percentage of the overall casualty figures is gradually decreasing as more ethnic Russians sign up to fight.
This decline is also particularly noticeable in the North Caucasus, where Bessudnov notes that the percentage of men being killed has fallen from 11 per cent of the total to two per cent.
bbcrussian.substack.com/p/ukraine-war-tuva-and-buryatia-pay-highest-price