When comparing the sizes of positive ions (cations) and negative ions (anions), negative ions tend to be larger.
This difference in size can be attributed to the number of electrons and the resulting electron-electron repulsion. When an atom becomes a cation by losing one or more electrons, it results in a smaller radius. This is because the loss of electrons reduces electron-electron repulsion and allows the remaining electrons to be pulled closer to the nucleus due to the increased positive charge from the unpaired protons.
On the other hand, when an atom gains electrons to form an anion, the addition of electrons increases electron-electron repulsion, which causes the electron cloud to expand. The larger size of anions is thus a direct result of this increased repulsion among the added electrons, which makes it harder for the positive nucleus to hold them close.
In summary, negative ions are larger than positive ions due to enhanced electron-electron repulsion in anions compared to the increased nuclear charge effect in cations.