Why does the thermal stability of Group 2 carbonates increase down the group?

Going down the group, Group 2 carbonates decompose at increasingly higher temperatures.This is because the Group 2 metal ion radius/size increases down the group while the charge stays the same.This means that going down the group, the charge density of Group 2 ions decreasesThis means that the large carbonate ion suffers less polarisation or distortion Therefore the ionic bond is more difficult to break