$\text{Zr}$ ($Z=40$) and $\text{Hf}$ ($Z=72$) have similar atomic and ionic radii because of:

Zirconium ($\text{Zr}$) belongs to the $4d$ series and Hafnium ($\text{Hf}$) belongs to the $5d$ series. Usually, atomic radius increases down the group. However, the $5d$ series follows the filling of $4f$ orbitals.The poor shielding effect of $4f$ electrons leads to an increase in nuclear attraction on the outer electrons, a phenomenon called lanthanoid contraction. This effect compensates for the expected increase in size, resulting in $\text{Zr}$ and $\text{Hf}$ having nearly identical radii.