Question

Why are large nuclei used for nuclear fission?

Answer 1

For large nuclei, the energy contained in their mass(E=mc^2) is larger than the energy
contained in the masses of the things they break up into. This means that the breakup(fission) is allowed energetically.

For smaller nuclei, the energy contained in their mass is smaller than the energy of the parts they might break into, and so they don't have the energy they need to break up.
So, no fission.