Indeed. To me the situation is like if USA were to invade Canada. They share a similar culture, language, plenty Russians know or have lived in Ukraine or viceversa.

The morale hit is enormous. Especially when these kids are told they're sent there to liberate it from the Nazi government and they see people that look, talk and shit exactly like them. How long can they follow the orders and trust the chain of command? And how long can the internal propaganda convince the Russians that their war is not a senseless massacre of cousins and friends?

It's not an exact science, 3:1, 5:1, highly dependent on morale, local conditions and so on. But defenders have an advantage assuming equal weaponry.

Russia has air superiority so that definitely upsets the balance, but so far they have not been able to use that very effectively. They did not even manage to protect their troop transport airplanes all that well, two of which got downed, and that really adds up in casualties.

It's a combat rule. Known from Sun Tzu, and later.