Zr (Z = 40) and Hf (Z = 72) have similar atomic and ionic radii because of :

The elements zirconium (Zr, atomic number 40) and hafnium (Hf, atomic number 72) belong to the same group (Group 4) in the periodic table.

Even though there are 32 protons separating them, their atomic and ionic radii are almost the same. This apparent anomaly is explained by the lanthanoid contraction.

The lanthanoid contraction refers to the gradual decrease in size of the lanthanoid (rare‑earth) series from lanthanum (La, Z = 57) to lutetium (Lu, Z = 71). As the 4f electrons are added, they do not shield the increasing nuclear charge effectively. Consequently, the effective nuclear charge felt by the outer electrons rises, pulling the electron cloud closer to the nucleus.

When the lanthanoid series ends at Lu (Z = 71), the next element, hafnium (Z = 72), experiences a much higher nuclear charge but is compensated by the poor shielding of the filled 4f ⁹⁹ electrons. This results in Hf’s outer electrons being drawn inwards, reducing its radius to a value very close to that of Zr, which does not have the lanthanoid contraction influence.

Therefore, the similarity in atomic and ionic radii of Zr and Hf is a direct consequence of the lanthanoid contraction, which offsets the expected increase in size due to the additional protons.