Excuse me if this is documented somewhere but I discovered it quite by accident.
We all know that files included in the 'Files' pane of the IDE are copied to the Files.DirAssets folder for use in the application. If they do not appear there then the files are not later available to the code.
I noticed that an .apk package had grown in size from 300k to 800k between compilation versions. Looking inside the .apk (rename it to .zip) I discovered that a bunch of artwork that I had copied to the project /files folder has been included in the /assets folder of the .apk, increasing it's size enormously.
I had innocently put these associated files there to keep the project tidy and altogether. They do not appear in the 'Files' pane and are not included or otherwise referenced in the project, yet they are packaged in the .apk !!!
Don't make my mistake!