This work deals with soft magnetic polycrystalline samples (YIG) specially prepared for fundamental investigations. These investigations have previously shown, at different times, that macroscopic magnetic properties of such materials could be interpreted by a simple spherical grain model. From theoretical study of this model, a first magnetization curve formulation is proposed. Excellent agreement with measurements in a broad range from weak fields to saturation is shown. In high‐quality materials with a negative magnetostriction constant, because of the very peculiar action on domain walls, parallel compression is shown to be a useful way to compare observation with theory. Quantitative study proves this specific role.