Why is there a significant difference between the radii of first and second row transition metals, where as no difference (or even a decrease) is observed between the second and third rows?

The answer stems from the lanthanide (or lanthanoid) contraction. In more detail, there is a steady decrease in the radii of atoms from Lanthanum through to Lutetium, despite the associated increase in the atomic number. As the atomic number increases, so does the Effective Nuclear Charge (Z_eff). Electrons are added successively to the 4f orbitals and experience a higher nuclear charge, due to the identical shielding effect of the core electrons, who’s number remains unchanged. This results in a successive decrease in radii, with the radius of Lutetium being significantly smaller than that of Lanthanum. This results in the alteration of normal periodicity trends and accounts for the similar radii (and hence similar chemical properties) of Zirconium and Hafnium, albeit their striking difference in atomic number (40 amu for the former compared to 72 amu for the later). In fact, Hafnium has a smaller atomic radius than Zirconium. Lattice and solvation energies are also similar, as these are radii depended. The lanthanide series starts after the second row of transition metals. The lanthanide contraction doesn’t, therefore, occur for the first and second row and an increase in size is observed, as a result of the addition of an additional shell of electrons, whilst the Z_eff remains approximately the same.