A smaller cation has higher polarization as it has a smaller size.
Polarization is the distortion of the electron cloud of an ion or a molecule by an electric field.
The formula for the electrostatic force between two charges
A smaller cation can get closer to the electron cloud of a nearby ion or molecule, so it can exert a greater attraction (distorting force) on those electrons.
Thus, a small cation has a greater ability to polarize nearby ions and molecules.