Down a group, ionic radii increases. But at the same time, due to decrease in charge density, the ions attract less number of water molecules. The smallest cation (with the same charges) having highest charge/size ratio attracts maximum number of water molecules.
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Compare the relative increase in ionic radii down a group to effective radii of hydrated ion, i.e., with attached whole water molecules. Clearly, there will be a decrease in radii of hydrated ions.