$\mathrm{CuCl}$ is more covalent than $\mathrm{NaCl}$ although $\mathrm{Cu}{}^{+}$ and $\mathrm{Na}{}^{+}$ have similar sizes and the same charge. The reason is that

Cations with an $18$-electron (pseudo-inert gas) core have poor screening by the d electrons, so the effective nuclear charge felt by the outer electrons is high. Their polarising power is therefore greater than that of a noble-gas-type cation of similar size and charge. Charge and size are the same here, and the anion is identical, so this is the cause.