If a free atom reacts to become part of a molecule, that atom gets smaller.
Work Step by Step
The atomic radius of a free atom would be the nonbonding atomic radius of that atom. When that free atom reacts to become a part of a molecule, then now its radius would be the bonding atomic radius of that atom. Since bonding atomic radius is smaller than nonbonding atomic radius, a transformation from nonbonding to bonding atomic radius means the atom has grown smaller.