@dutchie1,
I'm wondering how you have configured the calibre conversion settings for cover when you convert to epub? On the
Prefs - Common Options - EPUB Output page I have the following settings:
No SVG cover: UNCHECKED
Preserve cover aspect ratio: UNCHECKED
My android reading app of choice - Mantano Premium - does show the epub's page 1 cover completely edge-to-edge with the above settings, no matter what the actual width x height shape of the cover.
Opening the same epub in Moon+ has grey filler bars top/bottom or left/right. How big these are depends on the actual shape of the cover image. My android device is a 5" phone so what looks small to me may look huge on a 10" device
The difference between Moon+ and Mantano is not altogether surprising as those who choose Mantano over Moon+ often do so because Mantano has more faithful support for internal epub styling code. Moon+ has more customising features - 'you pays your money and takes your choice'.