Assertion: The radii of third (5d) series are virtually the same as those of the corresponding members of the second ( 4d) series. 

Reason: The filling of 4f orbital before 5d orbitals results in a regular decrease in atomic radii called lanthanoid contraction which essentially compensates for the expected increase an atomic size with increasing atomic number.

The process known as lanthanoid contraction occurs in the third series of transition elements. As a result, the 4f shell, which is filled with the 5d series, has electrons filling it. The filling of 4f orbitals before 5d orbitals causes their atomic radius to be extremely comparable to the elements of the second series, namely $\mathrm{Zr}=160$(atomic radius) and $\mathrm{Hf}=159$(atomic radius).

The 4f shell has a very weak shielding effect and is greatly attracted to the nucleus due to high nuclear charge.

Assertion is True, reason is True; reason is a correct explanation for Assertion.