Isotropic NdFeB-based sintered magnets, before and after a (Tb-doped) grain-boundary diffusion process, have been investigated by means of magnetic-field-dependent small-angle neutron scattering. Compared to the untreated sample, we found a reduced correlation length in the grain boundary diffused sample which, in an applied-field range of 8.5 T, varies between 30 and 35 nm – about 15% smaller than in the untreated specimen. This observation is related to the increased magnetic anisotropy field of the nucleation sites for magnetization reversal, and may be explained by a reduction in the effective defect size responsible for the magnetic inhomogeneities.