When a cation approaches an anion, the electron cloud of the anion is attracted towards the cation and hence gets distorted. This effect is called polarization. The greater the polarisation produced, more is the concentration of the electrons between the two atoms thereby decreasing the ionic character or increasing the covalent character. The covalent character of any compound in general depends upon size of anion, charge on ions and electronic configuration.
If two cations have the same charge and size, the one with pseudo noble gas configuration i.e. having 18 electrons in the outermost shell has greater polarizing power than a cation with noble gas configuration i.e having 8 electrons.
Electronic configuration of Cu+ : 2, 8, 18
Electronic configuration of Na+ : 2, 8
Therefore, CuCl is more covalent than NaCl although Cu+ ion (0.96Å) and Na+ ion (0.95Å) have same size and charge.