I'm talking about books I have boought. I paid money for these books and expect some quality.

Sorry, but this is nonsense. I'm not talking about photos, but about graphs, line art, tables. Using PNG you can create real small (in filesize) images, like 10-30 kilobytes. The size of the image (dimension) often doesn't matter that much. You can create the same image (black on white or line art with three or four colors) and resizing (less pixels) it won't make much of a difference in file size, nothing significant.

I cannot believe that publishers create small pictures because bigger pictures means that their book will be deleted later on. That argument seems far fetched.