While Russia is the world’s largest country in area its population is decisively lower than Nigeria’s. And Ukraine is much larger in area than Russia’s previous victims like Chechnya and the Republic of Georgia.
The February blitz tried to subdue a country that is larger in area than Indiana, Kentucky, Tennessee, Alabama, and Mississippi COMBINED. So, while Russia was able to flatten large swaths of urban terrain it could not simply overpower Ukraine.
Ukraine’s decisive advantage is the higher morale among its troops. The Ukrainians believe in their cause while the Russians soldiers are resentful for being sent to a meat grinder over false pretenses.
Russia’s population is similar to that of Java (part of Indonesia) - 140 million.
Though this is deceptive, quite a bit of that population number and certainly territory is not Slavic Russian. It’s Central Asian, Turkic & Siberian Asian. I doubt many of them are keen on dying for pan-Slavia.