Why both BeCO3 and BeSO4 are unstable to heat, while BaCO3 and BaSO4 are stable.Explain?

The decomposition of a salt is promoted by the formation of a new compound with high lattice energy. From BeCO3 and BeSO4, BeO will be formed by decomposition. The small doubly charged ions will result in very high lattice energy for BeO which will help to drive the reaction. (Reaction is also favoured by an increase in entropy since a gaseous product is formed).