How do you automate the process of figuring out which version is better? It's not safe to assume the smaller versions are always better, nor the inverse. Particularly for books with images, one version of the book may have passable image quality while the other compressed the images to jpeg mush. And there are considerations that are difficult to judge quantitatively, like the quality of formatting. Even something seemingly simple like testing whether a book's TOC is linked correctly entails a huge rats nest of heuristics and guesswork.