A magnetic field isn't the primary determination for an atmosphere, Earth is the only terrestrial object with an atmosphere that has a magnetic field. Venus is roughly the same size and has an atmosphere 2 orders of magnitude larger, and lacks any kind of intrinsic magnetosphere. Planetary mass matters much more in regards to atmosphere retention, as well as temperature and atmospheric composition. Titan is another good example, were it lacks both a magnetic field and mass, but the moon is so cold that it can maintain a relatively dense atmosphere.