Mostly because Russia doesn't even try to compete with the US Navy anymore. They have more immediate and important things to spend their money on.
They can afford to do this because they KNOW the US will never launch a first strike against them,which in effect means that no one is going to launch a first strike against them that can do them any real harm.
Truth to tell,we could stand a major cutback on the size of our Navy,also. We don't do it because of all the screams of outrage in states where there are large Naval bases,or where there are factories that manufacture items for sale to the Navy. Doesn't have a single damn thing to do with national security needs,and everything to do with jobs programs in states with influential congresscritters.