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

• 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.

• 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

• 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:
  1. Identify the largest contributors to app size.
  2. Determine if the dependency or asset is strictly necessary.
  3. Remove, replace, or optimize the identified component.
  4. Regenerate the Size Analysis JSON and compare the new build against the old build using the DevTools "Diff" tab.

Workflow: Estimating iOS Download Size

• 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

• 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

App Thinning Size Report.txt

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