[Gimp-user] Size of output files

[Alan Barnett <alansbarnett verizon net>]

I'm using gimp to increase the contrast of scanned documents.  The 
scanned images were combined into a multi-page pdf.  The images are 
music scores; the originals are RGB, but only grayscale information is 
important.
I opened the input pdf as layers with a resolution of 300 dpi, changed 
the brightness and contrast of each layer, and exported the images as a 
.mng file.  I then used ImageMagick to convert the nmg back to .pdf format.

I've done this with two different sets of images.  For the first set of 
images, the input .pdf files are about 2 Mb, and the output .mng and 
.pdf are both about 4 Mb; the process increases the size of the files by 
a factor of 2.

For the second set of images, the input pdf files are about 5 MB, and 
the output .mng and .pdf files are both about 45 MB, an increase of a 
factor of 9!

Any suggestions how to decrease the size of the output files? Any ideas 
about why the large difference in the size increase between the two sets 
of images.  (The first set were scanned by me on an EPSON 1640SU 
scanner, the second set were scanned by someone else on a scanner 
unknown to me.)

Thanks in advance for any suggestions.

Alan