The macOS Software Updater is extremely inefficient with space - you want plenty free or it might get stuck. As for the 15GB, 2.5GB must be used for the mandat

The macOS Software Updater is extremely inefficient with space - you want plenty free or it might get stuck. As for the 15GB, 2.5GB must be used for the mandatory Recovery partition as well as leaving room for the recovery partition to have extra room if necessary, because it can’t be easily expanded later. That leaves 12.5GB for your desktop Linux and any free space for your apps or files. You can use Expert Mode to go smaller with either macOS or Asahi but I’d say they are reasonable guidelines.