Hi,
It looks like a problem of image size of the ebook rather than kindle DX itself.
The native resolution of a kindle DX is 824x1200 pixels.
If in "Diary of wimpy kids", the resolution of a picture is lower than this, (example:560x750) that would explain it doesn't fill the whole surface of DX but only the third of that.
The resolution can be adjusted with some tools, like Mangle tool (
https://foosoft.net/projects/mangle/) but it requires time and effort.
The Comic/manga kindle conversion is a real issue.
PS: old thread for mangle DX:
https://www.mobileread.com/forums/sh...ad.php?t=48808