Answer:
Because d block is present on right side while s on the left. As we move from left to right size decreases.
Explanation:
Atomic radius
As we move from left to right across the periodic table the number of valance electrons in an atom increase. The atomic size tend to decrease in same period of periodic table because the electrons are added with in the same shell. When the electron are added, at the same time protons are also added in the nucleus. The positive charge is going to increase and this charge is greater in effect than the charge of electrons. This effect lead to the greater nuclear attraction. The electrons are pull towards the nucleus and valance shell get closer to the nucleus. As a result of this greater nuclear attraction atomic radius decreases from left to right. That's why d-block elements have smaller size as compared to s block elements because s-block is on left while d block is present on right side.
