Pressure dependent magnetotransport measurements have been carried out on a chlorine-doped Zn0.98Mn0.02Se epilayer with a carrier concentration below the critical concentration of the metal-insulator transition (MIT). The large positive magnetoresistance at low temperatures can qualitatively be related to the giant Zeeman splitting of the donor states. The increase of the resistivity and the magnetoresistance effects under hydrostatic pressure is mainly related to the increase of the electron effective mass in the conduction band. This increase of the effective mass is the reason for an increase of the donor depth and a corresponding shrinkage of the donor wavefunction, which in turn leads to a narrowing of the impurity band (IB). The splitting of this narrow IB, which results from an applied pressure, is the main reason for the enhancement of the large positive magnetoresistance effects under pressure.