How does lanthanoid contraction make the properties of second- and third-series transition metals similar?

The lanthanoid contraction is a phenomenon where the atomic and ionic radii of elements decrease steadily across the lanthanoid series due to the imperfect shielding of the nucleus by 4f electrons. This contraction significantly influences the properties of second- and third-series transition metals by keeping the sizes of atoms in the third series nearly identical to those in the second series.12

Because the 5d and 6s valence orbitals of the third-series elements are shielded poorly by the 4f electrons, the effective nuclear charge increases, causing these orbitals to contract. Consequently, elements like hafnium have radii almost identical to zirconium, which lies directly above them in the periodic table. This similarity in size, combined with analogous electron configurations, results in remarkably similar chemical properties and compound behaviors for pairs such as niobium and tantalum, molybdenum and tungsten, and rhodium and iridium. Without this contraction, one would expect a greater disparity in orbital sizes and chemical behavior between these series.21