Here is a brief excerpt from a chemistry textbook. Giving credit where credit is due, the book is Chemical Principles - Quest for Insight by Peter Atkins and Loretta Jones, this is from the 4th edition:
When salts crystallize from an aqueous solution, the ions may retain some of the hydrating water molecules and form solid hydrates such as Na2CO3·10H2O and CuSO4·5H2O. Both the size of the ion and its charge control the extent of hydration. The strength of the ion—dipole interaction is greater the smaller the value of r in Eq. 2 (that is, the closer the dipole can come to the center of the ion). Because of their stronger ion—dipole interactions, small cations attract the polar H2O molecules more strongly than large cations do. As a result, small cations are more extensively hydrated than large cations. In fact, lithium and sodiun commonly form hydrated salts, whereas the heavier Group 1 elements that have bigger cations – potassium, rubidium, and cesium — do not. Solid ammonium salts are usually anhydrous, or water free, for a similar reason: an NH4+ ion has about the same radius (143 pm) as an Rb+ ion (149 pm).
For ions of similar size we should expect hydration to be more important the higher the charge. We can see the effect of charge on the extent of hydration by comparing barimn and potassium cations, which have similar radii (136 pm for Ba2+ and 138 pm for K+). In the solid state potassimn salts are not hydrated to any appreciable extent, but barium salts are often hydrated. For example, barium chloride is found as BaCl2·2H2O, but potassium chloride is anhydrous. The difference can be traced to the barium ion’s higher charge. Lanthanum, barium’s neighbor, is both smaller (122 pm) and more highly charged (La3+); so we can expect it to have strong ion—dipole interactions and its compounds to be extensively hydrated. In fact, its salts include La(NO3)3·6H2O and La2(SO4)3·9H2O.
Ion-dipole interactions are strong for small, highly charged ions; one consequence is that small, highly charged cations are often hydrated in compounds.