A lot of people don't realize how much smaller the 21st century great power militaries are in pure manpower terms than the Cold War era, even for the US [2].
Additionally history is a huge factor here. Ukraine came under Soviet control as a consequence of the Russian Empire and this type of imperialism that had been rampant in human history up to this point largely died with World War Two and the onset of the nuclear age. European powers lost their remaining colonies (mostly) in the years following WWII.
The US has quite famously lost every war they've been in since 1945.
Ukraine as part of the USSR was really a vestige of a bygone era combined with a totalitarian government and a way stronger military than, say, exists now in Russia.
[1]: https://en.wikipedia.org/wiki/Soviet_Armed_Forces
[2]: https://historyinpieces.com/research/us-military-personnel-1...
Russian state TV is awash with pundits who are asserting the Russian empire should be re-created in Ukraine and the Baltics, and that Ukrainians are subhuman. There’s plenty of reason to believe Putin wants his empire.