This seems unlikely given the population disparity between the Russians and Ukrainians (which was similar then as it is today) and the casualty figures. I can't find actual estimates of the ethnic breakdown of the army, but there are breakdowns of casualties by SSR. Obviously SSR is not a perfect analogue of ethnicity, but the numbers are far enough apart that I think it does the job here. Roughly 65% of military casualties were from Russia, 15% from Ukraine. Ukrainians were one of only two groups to be overrepresented as a proportion of casualty figures relative to their population though, the other being Belarusians.