Why Android APK much larger when installed?

Question

I am installing an APK which weighs roughly 2MB. Once the application is installed it weighs 9MB. Is there a logical explanation for this?

Answer 1

The APK file-format is a compressed format.

As soon you install an app Android decompresses the App it because it performs some optimizations. Another reason is that it would be unfavourable to decompress it before every start for performance and battery-life reasons.

Some APKs bring some native libraries with them (for example many games or apps that were build with Delphi) which are also stored uncompressed. Escpecially this kind of apps will suffer from being enormous large when installed compared to Java-Only Apps.