Atomic radii of the transition series elements decrease initially with increase in atomic number on moving left to right in the periodic table due to which of the following reason?

Increase in nuclear charge

The decrease in atomic radii in each series in the beginning is due to an increase in the nuclear charge. At the same time, the addition of extra-electrons to (n-1) d orbitals also provide screening effect. Thus, the two operating effects oppose each other. In the midway, both the effects become nearly equal and thus, there is no change in the atomic radii.