Bitmaps are RLE compressed for storage. To see the effect of compression on a particular image, download the BMPload program and run it. Ignore the initial error (no SLCD attached) and add a bitmap. Then click on the bitmap name, and the information on the right-hand side will show the compressed size.
Note: If the bitmap is palletized using the SLCD palette, the 768 bytes of palette data does not need to be stored in the flash memory.