We revisit the HI size-mass (D_HII_-M_HI_) relation of galaxies with a sample of more than 500 nearby galaxies covering over five orders of magnitude in HI mass and more than 10 B-band magnitudes. The relation is remarkably tight with a scatter {sigma}~0.06dex, or 14 per cent. The scatter does not change as a function of galaxy luminosity, HI richness or morphological type. The relation is linked to the fact that dwarf and spiral galaxies have a homogeneous radial profile of HI surface density in the outer regions when the radius is normalized by D_HI_. The early-type disc galaxies typically have shallower HI 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_HI_-M_HI_ relation. This simple relation puts strong constraints on simulation models for galaxy formation.