Here's why:

* Atomic Number: Chlorine has a higher atomic number (17) than fluorine (9), meaning it has more protons and electrons.

* Electron Shells: Both elements have electrons in the same energy levels (shells), but chlorine has more electrons in its outermost shell. This makes the chlorine atom larger as the electrons occupy a greater volume of space.

* Shielding Effect: The increased number of electrons in chlorine's inner shells provides more shielding for the outer electrons, making them less tightly held by the nucleus and further extending the atom's radius.

In general, atomic size increases as you move down a group (column) on the periodic table and decreases as you move across a period (row) from left to right.