[dromanip]

I hope this is the right place to ask this (please refer me to the correct place if it is not), but it has come to my attention that epub files are sometimes unnecessary large. Say 30-40mb, I do not want to have too many of those on my e-reader. So I would love to have a tool to compress that, similar to for instance https://smallpdf.com/compress-pdf. Any suggestions?

I seems to me that the Technology section in the ebooks Club would be a better option to post your question, but I can tell you from experience that converting with Calibre from epub to epub usually reduces the size considerably

[dromanip]

Answering my own question:

epub files are just renamed zip files. So rename *.epub to *.zip and extract. Now look for the big files. Most likely it are the images which make the file unnecessary large. Reduce their size with a tool like Advanced JPEG compressor. Then re-create everything to *.zip, rename to *.epub and TADAAA: small .epub created!