Trends in solubility of group 2 nitrates
Solubility is one of the present scientific problems that nobody is able to explain  thoroughly. As you are speaking of Calcium compounds, look at the series of Calcium compounds made with halogens. $\ce{CaCl2, CaBr2, CaI2}$ are so soluble in water that they can be dissolved in less than their weight of water. $\ce{CaF2}$ should be similar. It is not. On the contrary, $\ce{CaF2}$ is one of the most insoluble compounds, because it forms the mineral called fluorite, which is one of the main sources of Fluorine atoms on Earth. If it would have been at least partly soluble, it would have been washed away by the rains a long time ago. Nobody is able to explain such a difference of solubility.

There are plenty of theories explaining the solubilities of some groups of substances using electronegativities, ionic or covalent radius, and other parameters, which work pretty well with a lot of substances. But there are always exceptions. The final explanation remains to be discovered.