Why do noble gases have comparatively large atomic sizes?

Explanation

The atomic radii of noble gases are by far the largest in their respective periods. This is due to the reason that noble gases (because they do not form molecules) have only van der Waals radii while others have covalent radii. Van der Waals radii, by definition, are larger than covalent radii. That is why noble gases have comparatively large atomic sizes.