Reflectivity and transmission spectra from wide GaAs and CdTe quantum wells (L=50-1000 nm) were studied in magnetic fields in Faraday geometry. The exciton Zeeman splitting was found to increase with the index N of the exciton quantization level. Since N is related to the magnitude of the exciton wave vector K, the observed effect means there is an increase of the exciton magnetic moment with increase of its kinetic energy.