Of the total population it may be small but the deaths are not a random sampling of the population. They are almost exclusively young men. If we define fighting age as 18-45 Russia has roughly 25 million men. 315k casualties is 1.26% of that population. That alone is a massive amount to lose in such a short period of time. Even before this Russia was in a demographic crisis.
And that 18-45 range is probably too generous. Soldiers are usually young, particularly front line infantry. In the US military like 45% of service members are under 25 and 65% are under 30. When you factor in the number of Russians who fled the country to avoid mobilization (~1 million) 10% of the men between 18 and 30 in 2021 being currently in the military, dead, or fled is a realistic estimate. That's crazy. Those are numbers we haven't seen since WWI. And the conflict isn't over, more will die.