[foopod]

Talking out of my ass, but since epubs often target mobile devices (kindle, kobo, etc - with limited storage) - publishers probably find that their books do better if they tend to be smaller in size. And images take up far more diskspace than text does in a digital copy.

Then when a user needs to delete books from a device they will start with the largest files. The longer the book is on the device the more likely they are to engage with it, etc.

[hollander]

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.

[foopod]

I did provide a warning of my nonsense. As you say though 10-30kb per image, in a 350+ page text book is already likely 7mb+ (assuming 1+ images pp). And that is absolute best case if the original images and optimization are working together. I would guess without any optimization for digital publication you could be starting from 100-200mb easily.