How to reproduce this problem in any Illustrator instance:
1. Create clean AI file with A4 page.
2. Place any pixel image (with size ~10 mb) embedded to this page. Save file, it will have size about ~10 mb)
3. Make this image as symbol and remove object from page.
4. Place symbol to page one time.
5. Save file. File size will be, eg, ~10 mb.
6. Now place same symbol to page more 4 times, total will be 5 images on page.
7. 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!
I just found that there is a difference between a Static and a Dynamic Symbol.
If you select the Symbol and choose Symbol Options... from the Symbol panel menu you can change the symbol from the default Dynamic Symbol to a Static Symbol.
All the link items will disappear and the file will be much smaller when saved.
That also explains why CS6 behaved different, Dynamic Symbols did not exist then.