I think you overestimate the Russians grossly. They screwed up in countless wars since then against small nations. They couldn't take out Czeczenia. They failed in Afghanistan. Why should they succeed in Finland.

Russia is capable of dealing with long drawn out battles on home turf. They have not displayed that great ability to invade. To be real, neither has really the US either.

You can't underestimate an opponent fighting on home turf for their own independence against an opponent, where the soldiers don't really see the reason why they are invading in the first place.

Also remember for whatever tech or equipment the Russians have. They have really shitty democratic traditions. They don't let their men work independently. History has shown again and again that top heavy armies are ineffective.

It is how Israel beat 3x bigger Arab armies, even when the Arabs had better tech.