The equation of formation of oxide, peroxide, and superoxide is

O₂^- + 1/2O₂→ O₂^2- + O₂→ 2O₂^-

Oxide peroxide superoxide

The increasing stability of the peroxide or superoxide, as the size of the metal ion increases, is due to the stabilisation of large anions by larger cations through lattice energy effects.

The size of the cations down group 1 increases from Li to Cs. Li has the smallest atomic and ionic size and has a strong positive field around it. Li⁺ combines with small anion called oxide ion (O^2-) with strong negative field and form Lithium oxide, Li₂O and Lithium peroxide in very small amounts.

Na⁺ ion is larger than Li⁺ ion and hence with less positive field around it. It combines with peroxide ion (O₂^2-) with less negative field and form sodium peroxide and sodium superoxide in small quantity.

K⁺, Rb⁺ and Cs⁺ ions are bigger in size with even lesser positive field around them. Hence they can stabilize the bigger superoxide ion (O^2-) with less negative field and form super oxides.