Which information is lost when saving .ai as hi-res pdf?

[arnoenzo]

I have a general question about the difference between .ai files and .pdf.

Example:
I have a 25Mb .ai file and I save it as a print-resolution .pdf. The pdf is appr. 10Mb.
When I read the pdf back into Illustrator, the file structure, layers, links and effects appear to be completely preserved. What is causing the difference in file size between .ai and .pdf, i.e. which information is omitted from the .ai file when saving to .pdf?
Why would I continue to save an .ai file, while I can save 15Mb for "the same file" in .pdf format?

[Ton Frederiks]

There is a lot of information lost when saving a PDF without checking the "Preserve Illustrator Editing Capabilities". To name a few: Brushes, Symbols, Appearances, Graphic Styles, Swatches, Layers.

So only do this with a copy and close the file after saving.

[arnoenzo]

Thanks for your prompt response, Ton.
I have "Preserve Illustrator Editing Capabilities" checked, so the difference in size comes from something else that is omitted.

[Doug A Roberts]

If you take your 25 MB AI file and Save As... then create a copy, is it still 25 MB?

[arnoenzo]

If we Save As .ai, then yes.

[Ton Frederiks]

Do you use images in your file?

When saving an Illustrator file the images get embedded (probably uncompressed) in the PDF part, even when they are linked.

When you save a Press Quality PDF file the images get downsampled and jpeg compressed which makes it smaller.

[John Mensinger]

None of this has any impactful meaning unless there is raster data in the file, but you haven't revealed whether there is. You do mention "resolution," but many posters here make erroneous references to that when speaking only of vector data, so I'd rather not assume.

If there is indeed raster data in the file, then that's where the potential for "loss" occurs, depending on the resolution resident  prior to and following Save As PDF, subject to the resolution and compression settings in the Save PDF dialog.

[arnoenzo]

John, there are both photographs and vector objects in the file.
Unless supplied with a .joboptions file from printers, we per default save our pdfs in the (modified with cropmarks, etc.) Press Quality preset.