I didn’t mean to imply that the W80 was the smallest nuke ever, just that it is the smallest currently available in the US arsenal.
I don’t believe the laws of physics permit a warhead to be miniaturized as you suggest. A pure fission plutonium only weapon still has to have about 8kg of Pu in order to even go boom. The MK 54 to which you refer had about that and only had a yield of 10-20 tons of TNT, about 1/1000 of the Hiroshima bomb, or alternately five OKC bombs.
If you’re just looking for a dirty bomb, a couple hundred grams of plutonium and an OKC size conventional explosive will contaminate a city.
Lets just say that the advances since the 1950s are not all going to being available to the civilian side. Especially advances that would pertain to micro destructive devices. To think that we have not learned how to make something smaller than the DC in the last 60 years, while seeing the incredible advances in Science and miniaturization during that time, is not a risk I'd bet on. Thanks.