To reduce the size of my EPUB, I use
Riot for optimize the picture of the book, especialy the cover.
I know that when integrating metadata Caliber my also updated the cover.
For JPG, okay.
But sometimes, a PNG cover, in 256 colors is more optimal without loss of quality (example with the attached image: Black and white, without or little gradation)
I optimize my image, put it manually in the book, then I update my metadata as well as the recorded size.
And I see that the size hasn't changed:
=> Caliber has reset the cover to TrueColors
(In the example image, the size increases from 226 KB to 95 KB)