Why do noble gases have comparatively large atomic sizes

Why do noble gases have comparatively large atomic sizes? 

Answer

Noble gases do not form molecules. In case of noble gases, the atomic radii corresponds
to van der Waal’s radii. On the other hand, the atomic radii of other elements correspond
to their covalent radii. By definition, van der Waal’s radii are larger than covalent radii. It is for this reason that noble gases are very large in size as compared to other atoms belonging to the same period.