We revisit the H i size–mass ($D_{\rm H\,{\small I}}$–M_H i) relation of galaxies with a sample of more than 500 nearby galaxies covering over five orders of magnitude in H i mass and more than 10 B-band magnitudes. The relation is remarkably tight with a scatter σ ∼ 0.06 dex, or 14 per cent. The scatter does not change as a function of galaxy luminosity, H i richness or morphological type. The relation is linked to the fact that dwarf and spiral galaxies have a homogeneous radial profile of H i surface density in the outer regions when the radius is normalized by D_H i. The early-type disc galaxies typically have shallower H i radial profiles, indicating a different gas accretion history. We argue that the process of atomic-to-molecular gas conversion or star formation cannot explain the tightness of the D_H i–M_H i relation. This simple relation puts strong constraints on simulation models for galaxy formation.