The present work aims to bring an original approach to the understanding of the polymorphic transition of RE2O3 from the low-temperature polymorph to the higher-temperature polymorph: from the cubic C to the hexagonal A form or from the monoclinic C to the monoclinic B form, for which the transition temperatures depend intimately on the rare earth element. The view proposed here, focusing on the lanthanide sesquioxide series, from Nd2O3 to Dy2O3, includes crystallographic and energetic considerations, using both bulk and surface energy calculations. After a complete description of the polymorphic filiations based on A, B and C unit cells described on simplified RE cation stacking schemes, it will be shown that the growth of the crystallite size as a function of temperature explains the existence of such transitions for these sesquioxides.