This has nothing to do with the conversion and everything to do with thedifferences in the azw3 and mobi viewers on the Kindle. As far as I know, the only way to guarantee an image takes up all available screen scpace in the azw3 format is to put it in an svg wrapper, which you will have to do by hand using the edit book tool. For an example of the svg wrapper, see the epub file here:
http://manual.calibre-ebook.com/conv...ormatting-demo