Please do a ls -l inside the epub's Images directory. You are using "du" and it is reporting that the disk space utilized to store the images is 81,980k bytes or roughly 80meg of images!
I am not sure what the empty ncx document is meant to show.
A simple ls -l done in the epub's Images/ directory when unzipped would help, as would a count of the number of images img tags in one file on average.
So please try the following in your directory of xhtml files or in the root of your epub:
grep "<img " `find ./ -name "*.xhtml"` | wc
That will give us a count of the number of image tags in all of your xhtml files and we can divide by the number of files to get how many images per page on average.
KevinH