Don't forget a number of games due to rom size the story is stripped or 'referenced' in the manual. Early D&D games were that way too.
I'd wager the space is mostly due to it being scanned manuals so images in a PDF wrapper, but haven't been cleaned up or optimized, or in other cases, OCR'd.
I've seen books made of scanned or screencapped images that range from 300k a page to over a meg per page; And most of it is unnecessary and is unoptimized. Other than illustrations pages should take up about 3k each (
and that's before compression). If i finish up my project of OCR stuff on another site. Course just cleaning the images up can result in greatly reduced size of said manuals, which i'd probably white/black threshold.
Did a cleanup example image for
@fake-kun on one image as an example and you can get an idea of what i'm talking about. The cleanup removes tons of variations of almost-black and almost-white and results in far better compression.