ADF is a raw image format that has all of the disk information, not just the usable storage area. You're seeing less because the standard disk format has overhead (bootblock, rootblock, block lists, etc.).
If you want to actually use the ADF file you don't just copy it to a formatted disk. You need to use a tool like DISKWIZ to write the image back to a floppy.
If you want to copy files back and forth between Amigas, then you'll probably have to compress the ADF file. LZX, LHA, etc. should compress them enough to fit on a floppy.
