The other thing that can happen occurs when a ePub with inefficient CSS usage is converted to MOBI. If the CSS for, say, the <p> tag contains 40 characters the bulk of that can get moved to each and every <p> tag in the html that is integrated into the MOBI adding bulk to the MOBI.
It more efficient to put the style in a <DIV> that spans all of the matching <P>'s leaving the <P> tags unstyled. This will reduce bloat in the MOBI conversion.
When converting ePub to MOBI with calibre, which doesn't embed the source in the output MOBI, the resulting MOBI files are sometimes slightly smaller, often slightly larger (~10%) and only occasional substantially larger. The latter generally being cases of poor CSS usage.
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