Spend some time looking at a map of Russia, and where it is located with respect to China. Now understand something the map doesn't show: most Russians live west of the Urals, in or near Europe.
Eastern Russia - Siberia - is a vast, forested land with mineral riches but few people. South of Siberia are two largely unpopulated nations - Mongolia and Kazakhstan, plus heavily populated China.
Tsarist Russian, followed by Soviet and now post-Soviet Russian generals have all worried about the world's most populous nation - China - coveting the huge unpopulated, resource-rich area to their north in Siberia.
See this article from The Moscow Times which explains those fears. Russia believes that their nukes keep China from annexing Siberia; they may be right.
