In this paper, based on the developed statistical-thermodynamic model, which is based on data on the local structure of the compound and taking into account the striction interaction caused by the large sizes of the Ba and K cations, the formation of ferroelectric phases in BaTiO₃ and KNbO₃ perovskites has been studied. Based on the modified eight-minimum model, it has been possible to qualitatively identify the factors that determine the features of the thermodynamic behavior of these crystals and to reproduce the process of formation of the whole set of phase states observed in BaTiO₃ and KNbO₃.