I don't know if anyone else has posted about this on this thread, but here goes. The Russian state in Tsarist times was expansionist wherever and whenever it could be. Cruel and ruthless too, of course. Under the Bolsheviks they were no longer expansionist, but they consolidated Russian rule more effectively by a continued programme of Russification and by settlement of Russians in non-Russian parts of the Soviet Union. After the state-initiated famines amongst Ukrainians, Volga Germans, Tatars and Kazakhs in the early 1920's, and again amongst Ukrainians, Kazakhs and Buryat Mongols in the 1930's, Russian settlers were imported in large numbers to replace the dead native inhabitants. This happened again when ethnic minorities were deported from border areas in the 1930's and 40's. So too when land was grabbed from Finland, Poland, Japan and Germany, with whole scale ethnic cleansing and the immigration of ethnic Russians to the border areas. This was all quite deliberate, looking to make the borders secure. Nobody at this stage foresaw the breakup of the Soviet Union, along the fictitious boundaries of soviet republics.
In the Far East and Siberia there has been a major retreat of Russian settlers back to European Russia, but there remain substantial numbers in the Baltic states, Byelorussia, Kaliningrad, Kazakhstan and eastern Ukraine. Whilst there is a Russian nationalist in the Kremlin we can expect more of this kind of thing in the future