Reflectivity spectra from GaAs and CdTe based wide quantum wells (widths L=50-1000 nm) have been studied in magnetic fields in Faraday geometry. The magnitude of the exciton Zeeman splitting has been found to grow with increase of the quantization index N for the exciton energy levels. Since N is related to the magnitude of the exciton wave vector K, the observed effect means that the magnetic moment of the exciton gets larger as the kinetic energy increases.