Question

why smaller cation can stabilise a smaller anion while a larger cation can stabilise a larger anion?

Answer 1

We know that electrostatic force of attraction is very important to bind the anions and cations in a molecule. Lattice energy depends on the electrostatic force of attraction between the anions and cations.

To have the maximum electrostatic force of attraction, the cations should touch the anions. But if the cation is small and anion is large or if cation is large and anion is small, we cannot have strong electrostatic force of attraction due to less overlap of the electron clouds of both. Therefore, stable lattice is not formed.

Due to form a stable lattice, larger cations stabilize large anions and smaller cations stabilize smaller anions.