Why the decrease in atomic radius amongst a series of transition elements is less when compared with representative elements ? 

Explanation

In transition elements differentiating electrons enter d-orbitals of penultimate shell. As the atomic number increases in a given series, though the nuclear charge increases, the electrons entering (n-1)d orbitals reduces nuclear attraction on the outer electrons. Hence atomic radius decreases slightly as we move from left to right in a series of transition elements.