Why chlorine has smaller atomic size in period 3?

Consider the elements in Period 3, neglecting the noble gas argon.

The outermost shell of electrons in ground state of the atom of each element in Period 3 is the third shell (M shell) . Therefore, the electrons in the outermost shell experience similar screening effect.

Across Period 3, the nuclear charges to attract the electrons in the outermost shell are increasing, and this leads to decreasing distances between the nucleus and the outermost shell. In Period 3 (neglecting the noble gas argon), chlorine has the shortest distances between the outermost shell, and thus the smallest atomic size.