In a period decrease in atomic size is less compared to a decrease in the atomic size of representative elements. This is due to added (n-1) d electrons providing a screening effect.

As we move from left to right

The atomic size decreases because

1- protons increase so Z increases

2- electrons in the same shell increase so the  shielding constant increases

If we conclude both factors there is an increase in Z(effective) so atomic size decreases

Z(effective) = Z - sigma

Sigma is shielding constant.

But in d block elements as we move from left to right the atomic size first decreases then remains constant and then increases. It is because at the half of the series sigma (shielding constant becomes nearly equal to Z in magnitude and after that, It dominates Z so at the Half of the series Z- effcetive remains nearly the same and then decreases or in other we can say poor shielding effect of d subshell is responsible for this behaviour.

Therefore, In a period decrease in atomic size is less compared to a decrease in the atomic size of representative elements. This is due to added (n-1)d electrons providing a screening effect.