Thank you for the post.
I may need a little more information to fully understand the steps to reproduce. To attempt to reproduce, I performed the following steps:
1. File > New From Starter Solution > Assets.fmp12 > Saved to Desktop
2. Moved the Assets.fmp12 to an iPad 3 with FileMaker Go 13
3. Added 15-20 pictures with the iPad's camera.
4. Moved the Assets.fmp12 from the iPad back to my Desktop
5. Get Info on the file (see screenshot below).
6. Deleted all the container images
7. Get Info on the file (see screenshot below).
8. File > Save a Copy as… > Assets Compact.fmp12 (compacted copy smaller)
9. Get Info on the file (see screenshot below).
Actual Result: File size reduced by 16384 bytes or slightly over 1%.
Please let me know if I am missing a step to reproduce.
And if you saved a clone of this file and then imported all records from the copy of assets where you deleted the images, what file size do you get then?