What insertion option was used? Was Store a Reference Specified?
External storage is a new feature that I am still exploring myself so I don't know if container fields with external storage, but without the "store a reference" option keep an embedded copy of the file in the container or not.
At first the picutures were still embedded in FMP12 after the import. Then I selected the containers in the Database Manager to store the container data externally in an open storage. Then I saw that files were created at [hosted location]/Files. I figured that after this action the database would shrink just to a few Mbytes, but it didn't.
Store a reference was not an option in this process, since I didn't import pictures in my solution.
UPDATE!! Hm, I just checked the database a later later and the file indeed shrinked to 10 MB! Problem solved. Perhaps FMP12 does some cleaning at night?? Thanks for thinking with me Phil!