Here is an example of how far they got with a weapon that was actually deployed by NATO in Europe 50 years ago. If the total warhead was 53 pounds, the nuclear material was probably some fraction of that. A lot of weight would have been used to keep the nuclear materials apart and allow the device to withstand the shock of being fired.
I am interested in the Astor Torpedo also. This was a nuclear torpedo carried by our submarines which had a yield about equal to Hiroshima. They said it was safe to fire, because the submarine would have its capability degraded by only 5%. My question was always for the submarine hundreds of feet underwater, and depending upon a host of mechanical devices functioning properly, which 5% was going to be degraded?
Davy Crockett (nuclear device)
https://en.wikipedia.org/wiki/Davy_Crockett_%28nuclear_device%29
Davy Crockett was a dirty bomb. No Nuke explosion.
Critical mass or implosion devises are not small.