Previous experimental studies have allowed us to observe peculiar localization effects of excitons in GaN/AlGaN quantum wells grown by MBE, as well as efficient nonradiative inter-well carrier transfers. In this work, we use the envelope-function approximation to calculate exciton energies and wave functions. We show that the typical spatial extension of the electron and hole can by itself explain the localization and transfer processes, mainly because, due to its large effective mass, the in-plane extension of the hole is smaller than the average distance between two aluminum atoms in the barriers.