I have the same problem regarding line spacing in AZW3 visualization on my Kindle 5, after conversion from an EPUB.
I have tried using different line height settings in Calibre, but this method doesn't seem to affect the final result. Even with different conversion settings, the line spacing is always the same, and I consider it too high even when set to minimum (on Kindle visualization settings). This is particularly noticeable if compared to the same book converted into Mobi format, instead of AZW3.
Is this a Kindle bug? Is there a way to "solve" the problem? Any suggestions or opinions?