This is an inherent problem with the kindle's handling of margins. Is your output profile set to Kindle? If so Calibre will handle the conversion a bit differently to address the problem, but some epub files have really large margins, so it will still look about like your screenshot.
To completely get rid of the margins you need to edit the epub in Sigil and get rid of the margins in the css. There are several other threads discussing this in more detail if you need more help. Try this one to start:
https://www.mobileread.com/forums/sho...de+left+margin