why ionic bonds are formed when degree of polaraisation is low and why not covalent bond?

According to Fajan, if two oppositely charged ions are brought together, the nature of the bond formed between them depends upon the effect of one ion on the other.
Whenever two oppositely charged ions cation and anion approach each other,it is found that the positive ion attracts electrons of the outermost shell of anion and on the other hand repels its positively charged nucleus.This results in the distortion,deformation or polarization of the anion.Degree of polarization is very important because if polarization is quite small,an ionic bond is formed,while if it is large is large,a covalent bond formed
The power of an ion (cation) to distort the other ion is known its polarization power and the tendency of the ion(anion) to get polarized by the other ion is known as its polarisability.Lower the polarization power or polarisability of an ion,greater will be its tendency to form a ionic bond.
Large positive Ion (Cation):-There will be smaller concentration of positive charge on a large area,the large cation has low polarising power,resulting in the formation of an ionic bond.