Sort every pixel on a scanned page by how dark it is, and they pile up
at the two ends. There is ink, and there is paper, and almost nothing
in between:
The 254 shades in the middle are paper texture and noise from the scan.
Throwing them away costs nothing, because there was nothing there — and
it means a pixel no longer needs to record which of 256 shades
it is, only whether it is black or white:
8 bits per pixel — one of 256 shades
1 bit per pixel — black, or not black
That alone is eight times smaller. A codec originally designed for faxes
(CCITT Group 4) is then used to compress long runs of white or black
together, rather than specifying the value of every pixel. Given that
most of a page of music is empty, that compression is where the remainder
of the space savings comes from.