Make an EPUB smaller by shrinking its images - here in this tab, with nothing uploaded.
Large EPUBs bounce off email limits and fill up readers. This shrinks the images in your browser and leaves the words exactly as they were.
Almost always images: photos saved far larger than any screen. Text is tiny by comparison.
At the default settings, not on an e-reader or phone - images are kept up to 1600 pixels. Lower the quality only if you need the file much smaller.
Only if they have no DRM. Copy-protected books are encrypted, and removing that is not something this site does. Books from Project Gutenberg, Standard Ebooks, Humble Bundle and most indie stores are DRM-free.