The magnitude of polarization or increased covalent character depends upon a number of factors. These factors are,
(1) Small size of cation:
Smaller size of cation greater is its polarizing power i.e. greater will be the covalent nature of the bond.
(2) Large size of anion:
Larger the size of anion greater is its polarizing power i.e. greater will be the covalent nature of the bond.
(3) Large charge on either of the two ions:
As the charge on the ion increases, the electrostatic attraction of the cation for the outer electrons of the anion also increases with the result its ability for forming the covalent bond increases.
(4) Electronic configuration of the cation:
For the two ions of the same size and charge, one with a pseudo noble gas configuration (i.e. 18 electrons in the outermost shell) will be more polarizing than a cation with noble gas configuration (i.e., 8 electron in outer most shell).