PSD files may contain a full resolution rendered (flat) bitmap for interoperability. It's probably enabled by default when saving. Other file formats like ODT a

PSD files may contain a full resolution rendered (flat) bitmap for interoperability. It's probably enabled by default when saving. Other file formats like ODT and ORA contain a rendered thumbnail (e.g. 256x256) just for fast preview in file browsers. Scaling down a couple of 20000x20000 images in a folder is no joke, and that's not an unusual size for digital artists.