The chart shows the number of Nazi dead, not the number of Soviets dead. It seems intuitive to me to measure the contribution toward stopping Nazi Germany by the number of Nazi dead. Presumably, the Nazis were going to keep fighting until they lost a certain amount of people (maybe losing ground is more relevant than losing people but the way they'd lose ground is by losing people anyway) which means that whoever put in more work to push them back and kill more of them contributed more to victory over Nazis than anyone else. That's the USSR.