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.