Cations is formed by the loss of electrons, hence proton are more than electron in a cation co electron are strongly attached by the nucleus and are pulled inward hence size decrease.E.g. Na - e- > Na+
on the other hand anion is larger than the parent atom. because anion are formed by the gain of electron. thus the number of electron are more than proton . the effective positive charge in the nucleus is less , so less inward pull is experienced. hence the size is expand.
Cl + e - > Cl-
Thats it.
