You're ignoring that Russia has to keep large percentages of its military on all its borders because Russia has no friends because Russia's leadership are jerks. No other government, especially the ones on Russia's borders, actually like the Russian government. China is a frenemy: Sooner or later, China's going to try to take all of Manchuria back, and Russia knows it. Whereas Ukraine is friends with all its neighbors except the one that bullies it: Russia. (As an aside, same with Armenia. Armenia's friends with everyone except the Turks in Turkey and Azerbaijan who've been progressively destroying Armenia for 800 years.) Which brings up another reason why your comparison isn't rational: Russia has troop commitments abroad, like between Armenia & Azerbaijan, that Ukraine doesn't have. Because Ukraine doesn't go around trying to bully its neighbors, but Russia does.