Why does mobi to mobi change file size?

[Vmurph]

I didn't post this question in the Calibre forum because it's not really about Calibre. The question is about conversion in general and my confusion about ePubs and Mobi files.

I was just fiddling around with Calibre, experimenting with different conversions and I noticed that converting mobi to mobi reduces the file size dramatically.

I started with a 3MB ePub (retail version) and converted it to mobi, and the mobi file was a little bit smaller, 2MB. But then I converted the MOBI to MOBI and the file size went from 2MB way down to 489K.

That's only a quarter of it's original size. What causes that kind of size difference? I'm guessing some data is stripped or altered during the process?

Just curious...

1.) What exactly occurs when a Mobi book is converted to Mobi? What is being changed?

2.) What happens to the file (content) when an ePub is converted to Mobi?

.

[pidgeon92]

3MB is a pretty big ebook file. Are there many pics in the original?

[Cyberman tM]

Maybe Calibre rewrote the way the book is encoded. I know it rewrites the HTML/CSS in ePub.
So if before every line had its own style definition, and afterwards there is a blanket statement "all lines look like this" it'll get much smaller.

[Vmurph]

Originally Posted by Cyberman tM

Maybe Calibre rewrote the way the book is encoded. I know it rewrites the HTML/CSS in ePub.
So if before every line had its own style definition, and afterwards there is a blanket statement "all lines look like this" it'll get much smaller.

But it was already converted to mobi by Calibre before I did that. The size only reduced a little when converting the ePub to mobi, but it wasn't until I converted the second time--the already converted mobi to a new mobi--that the size was reduced to a fourth of the first mobi.

Originally Posted by pidgeon92

3MB is a pretty big ebook file. Are there many pics in the original?

Nope, none...except for the cover. What's stranger is that it's actually a short. Only 52 pages! There's definitely something weird about it because there's no reason a 52 page book should be 3MB.

However, that's actually irrelevant to my question, because I've done the same thing with tons of other (normal size) books and the same thing happens. Books that were 800K, for example, will drop down to around 300K after being converting.

But, I've only noticed this drastic size difference when converting an ePub to a Mobi, and then converting that mobi to a new mobi.

So the biggest change--whatever is being lost, altered or omitted, enough to cause such a dramatic size difference--is not occurring during the (first) ePub to mobi conversion, it's occurring during the (second) mobi to mobi conversion.
.

[jersysman]

I really do think you can get a better response about the dynamics of the converting in the Calibre forum. Most of us here will be guessing and wondering just like you.

[Ripplinger]

I've seen books where the cover image was 7MB in size (yes, for just the cover). Idiotic waste of space. So it could very well be the one image causing it. Mobi files are generally larger than epub, even as much as 2x the size if you convert to both new and old format.

[pdurrant]

calibre conversions all go through an internediate ePub format. So the first transformation was ePub -> mobi. The second was mobi->ePub->mobi.

If you want to find out what changed, unpack the two mobipocket files and compare them.

[Vmurph]

Originally Posted by jersysman

I really do think you can get a better response about the dynamics of the converting in the Calibre forum. Most of us here will be guessing and wondering just like you.

You're probably right. If any of the mods are seeing this, would you mind moving this thread to the Calibre forum, please?

Originally Posted by Ripplinger

I've seen books where the cover image was 7MB in size (yes, for just the cover). Idiotic waste of space. So it could very well be the one image causing it.

Wow, I didn't realize that. How ridiculous. I didn't even think to check the cover image. Thanks for the tip. I'm going to check ALL of my covers from now on.

Mobi files are generally larger than epub, even as much as 2x the size if you convert to both new and old format.

I don't even know how to do that. My mobi settings are the default settings it came with, with the exception of putting the TOC and the beginning instead of the end when converting from ePub.

Does converting mobi to mobi remove one of the formats? I think I heard that a mobi converted from an ePub retains both formatting within it. So I'm wondering if converting a second time, mobi to mobi, removes one of the formatting?

Originally Posted by pdurrant

calibre conversions all go through an internediate ePub format. So the first transformation was ePub -> mobi. The second was mobi->ePub->mobi.

Ohhh, I see.

If you want to find out what changed, unpack the two mobipocket files and compare them.

Good idea. I've never unpacked a book and looked inside. How would I do that? Can Calibre do that, or would I need separate software?

[pdurrant]

Originally Posted by Vmurph

Good idea. I've never unpacked a book and looked inside. How would I do that? Can Calibre do that, or would I need separate software?

Get the KindleUnpack plugin - it's available through the calibre plugin system. It will unpack Kindle ebooks into files representing the contents.

[Vmurph]

Originally Posted by pdurrant

Get the KindleUnpack plugin - it's available through the calibre plugin system. It will unpack Kindle ebooks into files representing the contents.

Excellent, thank you.

[user743]

A better idea would be to to use calibre editor's compare feature:

Open one book with the editer.
Press the first menu, then "compare to book".
Choose/find the other book and it will show you all the differences.

[eschwartz]

Originally Posted by user743

A better idea would be to to use calibre editor's compare feature:

Open one book with the editer.
Press the first menu, then "compare to book".
Choose/find the other book and it will show you all the differences.

The Compare Books feature only works on EPUB and AZW3. Just like the editor itself.

[user743]

oh i forgot sorry