In case of inert or noble gases, the shells are completely filled with electrons and they are stable. As a result of this, due to the completely filled electronic configuration, the electronic repulsion in the noble gases are high. So the electrons tend to be away from each other to minimise this repuksion, leading to an increased atomic size of the noble gases.

Atomic radius of the noble gases are determined by van der waals radius, which is comparatively larger than ionic or covalent radii.