Why does calcium nitrate decompose in a similar way to magnesium nitrate, but require a higher temperature for decomposition?

Calcium ions (Ca²⁺) have the same charge as magnesium ions (Mg²⁺) but Ca²⁺ ions have a larger ionic radius (as ionic radius increases down the group). This means Ca²⁺ has a lower charge density than Mg²⁺. With a lower charge density, Ca²⁺ ions cause less polarisation and distortion of the nitrate (NO₃^-) electron cloud (and N-O/N=O bonds) than Mg²⁺ions making decomposition harder. Therefore, calcium nitrate requires more heat to be decomposed.