The performance of an optically pumped M_x magnetometer with miniaturized Cs cell at earth's magnetic field strength (50µT) is investigated. Operation using detuned laser light of high intensity is shown to be superior to the common resonant operation in terms of projected shot-noise-limited (50fT/sqrt(Hz)) as well as actually noise-limited sensitivity using a noise compensation method. The Zeeman light shift effect, emerging due to the off-resonant circularly polarized laser radiation and leading to a strong orientational dependence of the measurement, is suppressed by averaging two identical magnetometer configurations pumped with oppositely circularly polarized light. A residual heading error within the range of 14nT, limited by the present experimental characterization setup, was achieved.