Usually, atomic and ionic radii increase down a group due to the addition of a new electron shell. However, the radii of the second (4d) and third (5d) transition series are virtually the same (e.g., Zr ≈ 160 pm, Hf ≈ 159 pm). This is due to Lanthanoid Contraction.

The filling of 4f orbitals before the 5d series results in poor shielding of the nuclear charge by the 4f electrons. This causes a greater effective nuclear charge to pull the outer electrons closer, shrinking the size of 5d elements (like Hf) to be almost identical to their 4d counterparts (like Zr). Consequently, $Zr^{4+}$ and $Hf^{4+}$ have almost the same ionic radii.