@Themus - as I said it *really* depends on your settings, which makes it impossible for me to tell you exactly what you have done without them. Also which option did you choose to run on Modify ePub?
So in Preferences->Output Options->EPUB Output, what do you have set for "No SVG cover" and "Preserve cover aspect ratio"? On my machine I have those unticked. Then if I drag the Feedbooks book into calibre, run Modify ePub, tick the "Insert or Replace Cover" option, and click OK, click Yes on the dialog saying to confirm the replacement. The resulting ePub will show the full height and width cover with no white borders, because it is *not* attempting to preserve the aspect ratio. You can immediately see the difference in the calibre ebook viewer.
So follow those steps and you should see the same result.