> The amputation of a quarter of Russia's territory
Soviet Union is (was) not Russia, it was a multinational empire, similar to Roman or Mongol Empries. AFAIK, Russia itself did not loose much (any?) territory after 1989.
Unfortunately, many Russians don't seem to feel that way. They are not alone with that however: all former empires occasionally inspire the idea that their demise is an ongoing historical injustice that needs to be rectified. And in the echo chamber of a former-empire nation this train of thought rarely gets questioned.