The lanthanoid contraction is responsible for the fact that(Atomic numbers: Zr = 40, Y = 39, Nb = 41, Hf = 72, Zn = 30)

A regular decreases in the size of the atoms and ions in lanthanide series from  La3+ to Lu3+   is called lanthanide contraction. The similarity in size of the atoms of Zr and Hf is due to the lanthanide contraction.