Why do large cations stabilise large anions and small cations stabilise small anions?
For more detailed info, search "cation-anion radius ratio" or "Pauling's rules". Below is my review of this idea. 

This ratio is ${R_{C}/R_{A}}$ (C = cation, A=anion). 

Small cation (I mean, too small) attracts anions in such a way that they come too close to each other and repulsive force comes into play. This happens when ratio is &lt;0,155. So for larger anion, larger cation suits better. Ratio can be 0.155-1 for stable compounds, and this ratio also determines coordination number and type of void! (Check the picture below, recall that since &lt;0.155 ratio is for unstable compounds, you have empty "example" line there).