The 15MB were the images. When the feature was turned off, the images were still being included. This has been fixed in 8.2 (out last night).
The additional FFmpeg libraries (between 13 and 52 MB depending on the APK settings) are included if ANY of these are enabled:
- Allow saving as a video, combining the image and audio (in Verse on Image page)
- Share audio clips for the selected verses (in Book Collection > Sharing)
- Create and share video clips for the selected verses (in Book Collection > Sharing)
Regarding your suggestions:
Estimating Apk Size
When building an APK, it involves compressing the resources. We don’t know the compression ratio until building the app. Also, there are libraries that are downloaded during the initial build.
Download additional features
Based on Developer Program Policy, Android apps distributed through Google Play are not allowed to update themselves.
Although it might be technically feasible for apps distributed outside of the Google Play Store (I found an article how it might be done … from 3 years ago), maintaining versions of libraries for the different builds of apps generated by SAB would be very challenging and we don’t have the resources to do that. It could easily be block by the current OS and future OS updates. It has the potential to be exploited by unscrupulous developers and in my opinion it is not something we should consider (to save a few MBs).
What we should consider is enabling downloading content (not code) on demand. This has been on our feature list for a while. It would be good to have a way to download the backgrounds instead of having them always having to be packaged in with the app.