The ballooning storage issue was mentioned along with Flatpak, and is an issue as every single dependency is pulled in. AppImage excludes those dependencies which are "reasonably expected" to be on any Linux system. See the Application Sizes section here: https://askubuntu.com/questions/866511/what-are-the-differen...

Example: LibreOffice download size (source)

  - AppImage ~248 MByte  
  - Snap      463 MByte [July 2020 update]  
  - Flatpak   543 MByte

EDIT: formatting