This is because iodine has a higher atomic number than chlorine. The atomic number of an element is the number of protons in the nucleus of an atom. The more protons an atom has, the more electrons it has, and the larger the atom is.

Chlorine has an atomic number of 17, while iodine has an atomic number of 53. This means that iodine has 36 more protons than chlorine. This difference in atomic number means that iodine is a much larger atom than chlorine.