It will be the code in the page displaying the image. Odds are, the cover image is smaller and the code is simply putting it in the page without resizing it. The calibre viewer will show the cover page full size as it does not use the code. If it determines that the cover page is just the code to display the image, it simple displays the image.
If you change to cover page to use an SVG wrapper and have it fill the page, it should work better. There are options in the conversion or Polish to do this.
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