  Here I report the scaling relation between the baryonic masses and the scale
lengths of stellar discs from $\sim$1000 morphologically late-type galaxies.
The baryonic mass-size relation is a single power-law $R_\ast \propto
M_b^{0.38}$ across $\sim$3 orders of magnitude in baryonic mass. The scatter in
size at fixed baryonic mass is nearly constant and there is essentially no
outlier. The baryonic mass-size relation provides a more fundamental
description of the structure of the discs than the stellar mass-size relation.
The slope and the scatter of the stellar mass-size relation can be understood
in the context of the baryonic mass-size relation. For gas-rich galaxies, the
stars is no longer a good tracer for the baryons. High baryonic mass, gas-rich
galaxies appear to be much larger at fixed stellar mass because most of the
baryonic content is gas. The stellar mass-size relation thus deviates from the
power law baryonic relation and the scatter increases at the low stellar mass
end. Those extremely gas-rich low-mass galaxies can be classified as Ultra
Diffuse Galaxies based on the structure.
