Determining that is fairly easy, actually, because just about three whole Federal Districts lie to the east of a north/south line drawn at the Urals. The combined populations of the Ural, Siberia, and Far East Federal Districts amounts to about 37 million people. That's everything from a north/south line drawn at the Urals all the way to the Pacific Ocean. Total population of the Russian Federation is about 142 million people, so this comprises about 20-25% of the population.

Historically it was most likely more heavily weighed in favor of the West (East being more like 15-30% of total, rather than 40%), as the Eastern reaches of the Russian Empire were even more difficult to reach than they are now. . .no railroads, not a particularly reliable road system, and population centers few and far in between.

12 million of this is located in the Ural district (essentially central russia), with the rest scattered across the Siberian and Far East district.

EDIT: Screwed up population and names, corrected.