I know that SmartFpgEditor uses compression. I still use DIV sometimes for graphics work (the gimp is ok for modern stuff) but for pixelart nothing compares to the interface of DIV. I am used to it, eventhough I also use Grafx2, wich is a Deluxepaint clone. But Grafx2 not as user-friendly (very quirky amiga style interface) as DIV, and it doesn't support fpg/map formats. The ability save a file so that DIV can open it, is more or less a conveince for me, just for retro compatibilty.
Grafx2 is a nice color palette based editor, but in terms of interface friendlyness between Grafx2/D-paint and DIV is about 10 years of evolution.
I don't know if it's woth the effort to include an option in SmartFpgEditor or to extend in bennu's map_save / fpg_save functions. It depends on the demand. But for me it would only be a nice thing to have.
I am thinking about writing simple bitmap editor in bennu that supports map and fpg files with colorpalettes, since in the upcoming malvado 1.8 release I have created pixel editor for tile objects that editor uses. It is not a full graphics package, it can only draw pixels in a few select colors for the hardnessmap.