Within Europe, they had a massively superior conventional force, to the point that the US almost certainly would have initiated the use of nuclear weapons (tactical or strategic), after being overrun. Part of this is that they really only had to put their forces on their own border, and could support them with lines of communication entirely on land and within their own territory. The US had to pay a lot more to sustain forces in Europe, and keeping the Atlantic open during a major war would have been difficult.

At some point, nuclear forces are pretty much a binary thing -- either you have enough to guarantee a second-strike countervalue capability, or you don't. China seems to believe it can do this mission with ~200 warheads, today.