And historically, USSR always been placing military members from ethnic minorities into weakest, and most "cannon-fodderry" military units, predominantly foot infantry, and motorised.
Even ethnic Russians from more far away places were likely never to see any advanced military hardware.
Whereas the Baltic states were only conquered in 1940, and were considered extremely unreliable.
https://en.wikipedia.org/wiki/Treaty_on_the_Final_Settlement...
I'm not an expert and by any means I'm giving opinions here but I thought Crimea, South Ossetia and Abkhazia were territories were Russia at least has an historical or cultural excuse to claim. Again, not that I agree on those claims. On the other hand Kaliningrad was never part of Russia. It belonged to Poland and then to Germany. It was annexed by the Soviet Union in 1945.
I guess the reason Kaliningrad exists is that Poland and the Baltiks were in no means to fight for territory and Germany doesn't care.
The other territories vary... iirc Crimea was subject to a treaty in the 90s, etc. End of The day the relative power imbalances and international interest tend to rule the day.
In the aftermath of WWII, Germans in East Prussia were expelled, and the territory was split 50/50 between the USSR and Poland. No state other than Germany has a pre-WW2 claim, and they formally renounced that claim in the final peace treaty around 1990.