Originally Posted by Themus
So I am curious, what option in Calibre is fixing these covers so the thumbnail looks the right size in Aldiko and no white border shows up.
Probably the Preferences -> Conversion -> Common Options -> Page Setup -> Output Profile. Is it set to Tablet, as recommended for Android? The books at Feedbooks or wherever wouldn't necessarily be optimized for a specific device as well as calibre does when using the relevant conversion settings.