I don’t believe a true nuke explosion device can be that miniaturized.
Implosion is method to reduce the amount of core material, but it uses shaped charges to get past sub critical mass reaction.
Dirty bombs? Yes.
True nuclear explosion, no.
In fact, “miniature” nuclear bombs already exist, weighing less than 20kg and transportable by backpack. The soviets had even a design in a suitcase that can be lifted by hand. But it’s a classic fission bomb, with proven technics, no fancy laser-fusion-nanotech-whatever vaporware.