Reducing Flutter App Size

Contents

• Core Concepts

• Workflow: Generating Size Analysis Files

• Workflow: Analyzing Size Data in DevTools

• Workflow: Estimating iOS Download Size

• Workflow: Implementing Size Reduction Strategies

• Examples

Core Concepts

• Debug vs. Release: Never use debug builds to measure app size. Debug builds include VM overhead and lack Ahead-Of-Time (AOT) compilation and tree-shaking.

• Upload vs. Download Size: The size of an upload package (APK, AAB, IPA) does not represent the end-user download size. App stores filter redundant native library architectures and asset densities based on the target device.

• AOT Tree-Shaking: The Dart AOT compiler automatically removes unused or unreachable code in profile and release modes.

• Size Analysis JSON: The --analyze-size flag generates a -code-size-analysis_.json file detailing the byte size of packages, libraries, classes, and functions.

Workflow: Generating Size Analysis Files

Use this workflow to generate the raw data required for size analysis.

Task Progress:

• Determine the target platform (apk, appbundle, ios, linux, macos, windows).

• Run the Flutter build command with the --analyze-size flag.

• Locate the generated -code-size-analysis_.json file in the build/ directory.

Conditional Logic:

• If targeting Android: Run flutter build apk --analyze-size or flutter build appbundle --analyze-size .

• If targeting iOS: Run flutter build ios --analyze-size . Note: This creates a .app file useful for relative content sizing, but not for estimating final App Store download size. Use the Estimating iOS Download Size workflow for accurate iOS metrics.

• If targeting Desktop: Run flutter build [windows|macos|linux] --analyze-size .

Workflow: Analyzing Size Data in DevTools

Use this workflow to visualize and drill down into the Size Analysis JSON.

Task Progress:

• Launch DevTools by running dart devtools in the terminal.

• Select "Open app size tool" from the DevTools landing page.

• Upload the generated -code-size-analysis_.json file.

• Inspect the treemap or tree view to identify large packages, libraries, or assets.

• Feedback Loop:

• Identify the largest contributors to app size.

• Determine if the dependency or asset is strictly necessary.

• Remove, replace, or optimize the identified component.

• Regenerate the Size Analysis JSON and compare the new build against the old build using the DevTools "Diff" tab.

Workflow: Estimating iOS Download Size

Use this workflow to get an accurate projection of iOS download and installation sizes across different devices.

Task Progress:

• Configure the app version and build number in pubspec.yaml .

• Generate an Xcode archive by running flutter build ipa --export-method development .

• Open the generated archive (build/ios/archive/*.xcarchive ) in Xcode.

• Click Distribute App and select Development.

• In the App Thinning configuration, select All compatible device variants.

• Check the option to Strip Swift symbols.

• Sign and export the IPA.

• Open the exported directory and review the App Thinning Size Report.txt file to evaluate projected sizes per device.

Workflow: Implementing Size Reduction Strategies

Apply these strategies to actively reduce the compiled footprint of the application.

Task Progress:

• Split Debug Info: Strip debug symbols from the compiled binary and store them in separate files.

• Remove Unused Resources: Audit the pubspec.yaml and assets/ directory. Delete any images, fonts, or files not actively referenced in the codebase.

• Minimize Library Resources: Review third-party packages. If a package imports massive resource files (e.g., large icon sets or localization files) but only a fraction is used, consider alternative packages or custom implementations.

• Compress Media: Compress all PNG and JPEG assets using tools like pngquant , imageoptim , or WebP conversion before bundling them into the app.

Examples

Generating Size Analysis (Android)

Generate the size analysis JSON for an Android App Bundle

flutter build appbundle --analyze-size --target-platform=android-arm64

Splitting Debug Info (Release Build)

Build an APK while stripping debug info to reduce binary size

flutter build apk --obfuscate --split-debug-info=build/app/outputs/symbols

Reading the iOS App Thinning Size Report

When reviewing App Thinning Size Report.txt , look for the specific target device to understand the true impact on the user:

Variant: Runner-7433FC8E-1DF4-4299-A7E8-E00768671BEB.ipa Supported variant descriptors: [device: iPhone12,1, os-version: 13.0] App + On Demand Resources size: 5.4 MB compressed, 13.7 MB uncompressed App size: 5.4 MB compressed, 13.7 MB uncompressed

Interpretation: The end-user download size (compressed) is 5.4 MB, and the on-device footprint (uncompressed) is 13.7 MB.