How Ukraine could destroy Russia as a country?

I am have seen the declarations like For Russia this conflict is about preservation not only of its elites, but the country itself, source. Looks that there are people who think that Russia can be destroyed completely as a country if Ukraine wins.

How do they envision this? Ukraine has the 44.13 million population while Russia has 144.1 million. The demographic proportions are likely comparable. Is it really so that they are afraid of Ukraine advancing towards Moscow, Saint Petersburg ... and taking them? Or do they think this may happen some other way?