More shielding = larger atomic radius

Shielding is when electrons in the inner electron shells of an atom can shield the outer electrons from the pull of the nucleus.

The nucleus can pull the outer electrons in tighter when the attraction is strong and less tight when the attraction is weakened.

Therefore, the more shielding that occurs, the less attraction there is between the outer electrons and nucleus, so the the further the electrons in the outer shell can spread out. This means the atomic radius will be larger.