Russian army is a giant, they are yet to completely mobilize their forces.
Problem is, Russian army is so very regular, they can break US army into pieces using their giant reserves of cruise missiles. They can rain a hell of hypersonic BMs over the heads of Americans. That's the truth about regular Russian army. But in case of Ukraine, it is like fighting amonst a lion and a mongoose. The mongoose can delay its death but not for too long.
Ukraine has lost lands and despite receiving 1/3th of US ATGM reserves, for example, they cannot push Russian army back from its territories.