To grasp it better, we must first learn about Lanthanide Contraction.
The shielding order in the same shell is s>p>d>f.
As a result, s and p subshell electrons have high shielding, but d and f subshell electrons have weak shielding.
Because shielding is inadequate, the effective nuclear charge is concentrated in the d and f orbitals.
Despite the addition of a new shell to Hf, the size of both is almost identical, which is not the case for s and p block components.
The addition of a new orbital always results in an increase in the size of the atom, which is not seen here.
Thus, despite their varied ages, Zr and Hf have identical characteristics.
Final Answer:
Because of lanthanide contraction, Zr and Hf have comparable characteristics.
Because the electrons in the f-subshell do not have a good shielding effect, as the atomic number of effective nuclear charge increases, the size of Hf and Zr becomes almost identical and so both have comparable features.