I occasionally run into ePubs that are larger in MB than I think they should be (or, at least, larger than I'd like for uploading to my Kindle Fire via Amazon's webpage or storage in my HD). I've tried a couple of the Calibre-based fixes but nothing worked particularly well.
I did find a website that does very good compression and doesn't seem to adversely affect the appearance of the ePub. It's
https://tools.nooapps.com/en/compress-epub.html
I have no association with the site's owner. Just wanted to contribute something helpful. Please let me know if it works for you (or not).
Thanks,
Barry