Originally Posted by dwig
A third example is Lysistrata by Aristophanes. Here, all of the illustrations, except the cover, are GIFs in the ePub. The MOBI version is nearly 3x the size of the ePub, 632k vs, 1853k.
What appears to be happening is that it may not even matter what file type or size you put in. I think the conversion is taking the actual image pixel data and saving it as a jpeg with the highest quality compression. Here's why I think this:
I took my original EPUB with the larger gif file sizes--roughly 9MB. I ran that through Calibre and it came out to the same 15MB MOBI as when I converted the 4MB EPUB. I then did MOBI to MOBI on that file and it dropped down to 11MB just like the 4MB EPUB did.
It's just crazy that a 9MB and 4MB version of the same EPUB would both come out to be the same 15MB size MOBI.