Thank you for your post.
FileMaker Pro 12 can store files as embedded or by reference only.
If the goal is to store only a reference to the file being inserted into the container field, the user should be checking the box “Store only a reference to the file” before clicking “Insert.”
If that was the process followed, you might try making a compacted copy of the file to see how much the file size is reduced.
If the following does not resolve the issue, please let me know and we can discuss next steps.