Exciton reflectivity from GaN/AlxGa1-xN quantum wells (QWs) shows broad peaks that are difficult to analyze within a conventional single-free-exciton model. We have applied a new formalism that allows us to separate numerically radiative and inhomogeneous broadenings of an exciton resonance comparing the Fourier-transformed reflection spectra with calculated time-resolved reflectivities. We have found the exciton oscillator strength to decrease dramatically with the increase of the QW width in GaN/Al0.07Ga0.93N system. The collapse of the oscillator strength is a manifestation of the polarization field effect, as confirmed by our variational calculation. We find that only excitons in very thin quantum wells have an oscillator strength exceeding that of the exciton in bulk GaN.