@Dharma
Dunno. Pretty sure that's one of those areas where the classified part of this comes into play. I do know that the critical mass of whatever is somehow proportional to density...so one would assume that the denser the isotope they use at the highest purity would require a smaller sphere of fissile stuff. So the weight is probably coming from the conventional explosives et al. If they could get it down to 50 pounds in the 1960's, can't imagine why they couldn't take it down even further by now.
It was already a backpack nuke. A suitcase nuke doesn't seem like a stretch after half a century.