The lanthanide contraction is responsible for the fact that:

Lanthanide contraction refers to the steady decrease in the size of atoms/ions of lanthanide elements with increasing atomic number.This contraction compensates for the expected increase in size when moving from the $4d$ to $5d$ transition series.As a result, pairs like Zirconium ($\text{Zr}$) and Hafnium ($\text{Hf}$) have almost identical atomic radii ($\approx 160\text{pm}$) and very similar chemical properties.