covers displaying too small in kindle fire HD8

[merlin7777777]

I have a kindle fire HD8 7th gen. I have updated some covers by downloading images from the net and adding them in calibre by using the metadata/covers browse function.

When I open the book in the kindle, the cover displays too small, i.e. it doesn't fill the screen. If I double tap the image it zooms to fill the screen and the resolution is acceptable.

How can I get the covers to display full screen? Do I need to convert them to a certain size or is there a 'display cover full frame' option or something?

any help would be gratefully received.