my understanding to is the mass only drops if sub atomic particles are destroyed during the fission. otherwise it's just re-arranged into different atoms. u235 is split into other lighter materials but total mass of byproducts is same.
In a way, yes.
Fission does release energy with loss of mass.
A black hole is not a fission reactor. It fuses matter, in essence two atoms are forced to occupy the same space. The mass increases.
The heat that is created under such extreme pressures is like at the other end of the scale from fission heat.
So if the moon developed a really strong gravity for some reason would it shrink visually?
Lets say a tiny chunk of neutron star were embedded into the moon and stop (not likely but for all intentions of discussion).
Not only would the neutron star matter pull all the moon's matter towards it, thereby shrinking the moon. It would also cause gravity issues on Earth. Depending on the amount of mass in question, the Earth could slowly spiral onto the moon. Likely our atmosphere first followed by our water and eventually the dirt and rock. All the while the Earth would be moving closer to the moon.
It would also affect the orbits of all the material in the solar system, including the distance of the Sun. With enough mass, the moon could pull all the matter in the solar system to it creating a new star with much higher mass.