Wu P.-F.PO-FENG WU2022-11-112022-11-11201800358711https://www.scopus.com/inward/record.uri?eid=2-s2.0-85048147453&doi=10.1093%2fmnras%2fstx2745&partnerID=40&md5=6664b2c22a4df61f6ab386ed166bcc1bhttps://scholars.lib.ntu.edu.tw/handle/123456789/624932Here I report the scaling relationship between the baryonic mass and scale-length of stellar discs for ~1000 morphologically late-type galaxies. The baryonic mass-size relationship is a single power law R* ∝ Mb0.38 across ~3 orders of magnitude in baryonic mass. The scatter in size at fixed baryonic mass is nearly constant and there are no outliers. The baryonic mass- size relationship provides a more fundamental description of the structure of the disc than the stellar mass-size relationship. The slope and the scatter of the stellar mass-size relationship can be understood in the context of the baryonic mass-size relationship. For gas-rich galaxies, the stars are 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 relationship thus deviates from the power-law baryonic relationship, and the scatter increases at the low-stellar-mass end. These extremely gas-rich low-mass galaxies can be classified as ultra-diffuse galaxies based on the structure. © 2017 The Author.Galaxies: fundamental parameters; Galaxies: spiral; Galaxies: structureStars; Galaxies: fundamental parameters; Galaxies: spirals; Galaxies:structure; Late-type; Mass disk; Mass size; Power-law; Scaling relationships; Stellar disks; Stellar mass; Galaxiesjournal article10.1093/mnras/stx27452-s2.0-85048147453