Class-D Nuclear Warheads are considerably more compact than any nuclear device currently known and qualify as a "suitcase nuke". Judging by the total instant destruction of the Cylon Basestar in Kobol's Last Gleaming, they would appear to have a yield in the megaton range.
This would indicate that a Class-D Nuclear Warhead is a fission-fusion bomb utilizing a very high degree of efficiency of matter to energy conversion. Considering the relatively small size of the device that Baltar delivered to Gina, the core of the warhead (typically a plutonium bomb "trigger") is extremely small and lacks adequate shielding. This would indicate that either the amount of fissionable material (plutonium) is quite small or that the shielding is far better than anything currently produced on Earth. Also, in its stripped down form, the trigger lacks the fusion material for further yield, which reduces the power of the bomb by several magnitudes.
Another possible option would be that something other than plutonium is used to create the initial fission explosion. This is not likely as the show typically uses grounded physics and science where possible, and plutonium is by far the most likely candidate.
Notes
- This article is assuming that the shown nuclear warheads are Class D.
- In the United States military, Class-D is the designation of a light-weight Thermonuclear weapon. The W-28 warhead in the B-28 TN bomb is an example of this type.