heavy file size goes magicly light when saving to IDML file - why?

Your indesign file is a bunch of code.

Saving to IDML changes it to an InDesign MarkUp Language, which only gives relevant code information.

Once you open the IDML in InDesign it rebuilds the file.

If opening in an older version of InDesign, the newer features that you may have used will be reduced back to their basics. For instance, if you used Span/Split columns, and opened the file in a version of InDesign without that version, you would see some strange layouts in those areas that need to be tidied up.

If opening in the same version of InDesign it should reopen as it was.

As the file is just a bunch of code - each time you save your InDesign file you are adding to that code, which in turn increases the file size. Even if only small amounts of changes.

It is saving other information in the background. You can view this information in the file Information - open an InDesign file and got to About InDesign and hold down the CMD/CTRL key and it will show you a file info dialog.

Each section contains quite a lot of information.

Roundtripping to IDML will remove a lot of that information - which is only miniscule - but shows that the file is rebuilt and non-essential code is now defunct and removed.

Which leads to smaller more efficient files.

It's also a good way to fix issues with a file that is behaving weirdly.

I would think that this has a lot to do with the fact that IDML does not include thumbnails of the images used! I don't think you can tell the INDD to not include thumbs... someone might know that.

Hi,

This is a good point. IDML files are exchange files. To avoid compatibility issues, a lot of advanced features will not be included to retain only the core items of your document.

Which means it will be lighter but you could loose the possibility to edit some parts of your document.

To be as large as you said, your file must have the images embedded instead of linked. I think Adobe assumes we all have modern machine that can easily handle and store such large files.