[FelixKrull]

Hi everyone,

I’m running into a pretty extreme file size issue when converting an EPUB to AZW3 using Calibre.

Example:

Original EPUB: 14 MB (contains lots of pictures)
Converted AZW3: 140 MB

So the output file is about 10× larger, which seem way beyond normal differences.

From what I understand so far:

EPUB is essentially a ZIP container, so its size is heavily compressed
During conversion, images and content may be extracted and stored less efficiently
There might be duplication of resources (HTML, images, fonts?) in AZW3
PNG images in particular might be a problem if they are not recompressed

However, even considering all that, a jump from 14 MB to 140 MB feels excessive.

What I already checked/tested:

The option “Do not compress images” is NOT enabled
Tried different output profiles (Kindle Paperwhite etc.)
No obvious difference in settings affecting file size

Questions:

1. What are the most common reasons for such a drastic size increase?
2. Does Calibre re-encode images at all, or mostly pass them through unchanged?
3. Is there any built-in way to force proper image compression (e.g. reduce JPEG quality or convert PNG → JPEG)?
4. Could internal duplication in AZW3 (KF8 structure) explain such a large factor?

If needed, I can inspect the EPUB contents (images, formats, resolutions), but I’d like to understand first whether this behavior is expected or indicates a specific issue.

Any insigts or best practices would be appreciated!

Thanks in advance.