Russia today is the largest country in the world (by a fairly wide margin). That is true after the other 15 or so "Socialist Republics" left and went their separate ways in 1991, leaving Russia with a bit more than three quarters of the former Soviet land area. If physical size were the main determinant of power, Russia would not have lost all that much.
Another measure of Russia's lost power is the decline of population. Here, the loss was more serious, because Russia was left with only about half of the population of the former Soviet Union. And today's Russia seems to be a shadow of its "former" (Soviet) self.
So is one fourth (based on land) or one half (based on population) a better (rough_ guess of the decline in Russian power when the Soviet Union broke up? Are there any other other measures of power (e.g. GDP, nuclear warheads, troop strength, Nobel Prize winners, or any other relevant power source) that might provide a better estimate of Russia's loss?