I appreciate that, but the force which the vaporised material exerts on the ship baseplate is still a function of the magnitude of the blast. Therefore real limits must be placed on the maximum explosive strength, based on the design of the ship. Besides anything else, fuel constraints will determine the optimal explosive magnitude (since each explosion will have a associated cost in fissile materials).
I don't really see what the argument is here

A series of small controlled nuclear explosions are still an application of nuclear fission / fusion with a practical purpose in mind - no different to "civilian" applications of nuclear fission / fusion (for power generation).
This is a big step from creating uncontrolled nuclear explosions which have no purpose other than destruction. While it is clear that such an application is possible, the specific nature of an alien intelligence or culture would determine whether they would even consider building such a device. Devoid of the need for conflict between members of their species, and without any large-scale external threats, they may not have any need for such a device. Hence curiosity as to why we would construct them...