That just sounds like poor image file management. If I were working on a multi-layered PS design I would save the file at each major stage of design. Then I'd flatten the PS image, so all layers become one layer, saved as a different file name, before I'd move on to the next stage of multi-layer development. In the end I might have a half dozen separate files, each containing a flattened version of the previous stage. There would always be a version of unflattened at various stages so I could go back and edit if I need to. The final map image might be 10 or 15 MB total. Besides whoever the final user of the map, has no need of a multi-layered design. Flatten it all, and the file size becomes small, relatively speaking.