Comment by Llamamoe

Depending on how important it is for you to maintain original quality, I have in the past had good luck with a combination of prerendering complex content, reducing the DPI and colour depth of images, and recombining them back into PDFs, depending on the file.

You could probably easily automate identifying different editions of the same content, and e.g. only keep an epub with small images, rather than the other 6 and 3 more PDFs as well.