Why is the release .apk smaller than th debug .apk

Question

In my app's example, the debug apk is 20Mbs (from 13Mbs, after upgrading the Gradle version and Gradle Plugin version), and the release apk is just 5Mb. Why is that?

In the build.gradle file, the buildTypes part has remained unchanged, so there hasn't been any optimizations / shrinking from the R8 compiler or from ProGuard.

android {
    ...
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

RELEASE:

DEBUG:

You can see the big difference in size in the lib folder, among others.

Answer 1

There are more optimizations happen during the release builds besides ProGuard and R8. According to the documentation https://developer.android.com/studio/build/shrink-code.html

When you build you project using Android Gradle plugin 3.4.0 or higher , the plugin no longer uses ProGuard to perform compile-time code optimization. Instead, the plugin works with the R8 compiler to handle the following compile-time tasks:

Code shrinking (or tree-shaking): detects and safely removes unused classes, fields, methods, and attributes from your app and its library dependencies (making it a valuable tool for working around the 64k reference limit). For example, if you use only a few APIs of a library dependency, shrinking can identify library code that your app is not using and remove only that code from your app. To learn more, go to the section about how to shrink your code.

Resource shrinking: removes unused resources from your packaged app, including unused resources in your app's library dependencies. It works in conjunction with code shrinking such that once unused code has been removed, any resources no longer referenced can be safely removed as well. To learn more, go to the section about how to shrink your resources.

Obfuscation: shortens the name of classes and members, which results in reduced DEX file sizes. To learn more, go to the section about how to obfuscate your code.

Optimization: inspects and rewrites your code to further reduce the size of your app's DEX files. For example, if R8 detects that the else {} branch for a given if/else statement is never taken, R8 removes the code for the else {} branch. To learn more, go to the section about code optimization.