Regression: Illustrator CC saves each instance of symbol in .ai file separately
How to reproduce this problem in Illustrator СС:
Create clean AI file with A4 page.
Place any pixel image (with size ~10 mb) embedded to this page. Save file, it will have size about ~10 mb)
Make this image as symbol and remove object from page.
Place symbol to page one time.
Save file. File size will be, eg, ~10 mb.
Now place same symbol to page more 4 times, total will be 5 images on page.
Save file and check size - it will be increased to ~50 mb.
As result, you will have ai file, containing only 5 copies of one image, with ~50 mb size instead of ~10 mb!
Same steps on Illustrator CS6 works well - file size will not be increased at 5x times. So seems there is regression.
Alexey Korepov, that is correct. In a PDF, repeating content can be stored only once as a Form XObject. Static Symbols in Illustrator behave like this. Dynamic Symbols are unfortunately stored as multiple Form XObjects, defeating the purpose.
Alexey Korepov commented
As this happens only with Dynamic Symbols, seems PDF format allow placing exactly same object in several places of page without storing full copy of this object in file.
So the source of the problem is that Illustrator don't check Dynamic Symbol instances on page, if they changed or not.
Most of dynamic symbols placed on pages without changes (especially because Dynamic symbol type is selected by default in Illustrator), so if we add this check to Illustrator, it will reduce size of many files for users.
Can Adobe add check for changes in Dynamic Symbols and store them as Static Symbols in file, if no changes applied to symbol instance in document?
Alexey Korepov commented
So PDF format don't allow placing exactly same object in several places of page without storing full copy of this object in file for each place??
I believe this happens when an AI file is saved with PDF compatibility turned on. Basically, there's the AI data in the file, and a complete duplicate saved in PDF format. Try turning PDF compatibility off and see if that helps with file size. The down side to this, and a HUGE GRIPE I have with this feature, is that you will not be able to place AI files into InDesign documents without PDF compatibility turned on - INDD doesn't support AI files, only PDF. Unfortunately for those of us who use INDD in our work flow, this means that using symbols and linked images/content offer absolutely NO advantages. I've submitted a feature request to both AI and INDD feedback forums before but it's been largely ignored.
Seem to happen with the default option Dynamic Symbol.
Static Symbols work as expected.
See also this thread: