Bundle Size

Bundle size describes how big your app is. In React Native the word covers two different things, and conflating them is the most common source of confusion:

• The JavaScript bundle — the file Metro produces from your code and npm dependencies, shipped as Hermes bytecode in release builds.
• The binary size — the full .ipa or .aab: the JavaScript bundle plus the React Native runtime, every native library your dependencies pull in, fonts, images and other assets.

The JavaScript bundle matters for startup time, because more code generally means more to load and run at launch. The binary matters for download size, store limits on cellular downloads, and conversion: users on slow connections or small phones abandon large installs.

What users actually download is usually smaller than your build output. Google Play generates device-specific APKs from an Android App Bundle, and the App Store thins iOS builds per device. Check the sizes reported by App Store Connect and the Play Console rather than the file on your disk.

Practical levers:

• Audit dependencies. A single convenience package can drag in a large native SDK or a whole utility library.
• Import only what you use, and avoid barrel files that re-export an entire package.
• Enable R8 shrinking for the Android native side.
• Compress and correctly size images; ship vector icons rather than PNGs at three densities.

A frequent gotcha is assuming the JavaScript bundle is tree-shaken like a web build. Metro has traditionally included every module you import, so unused exports still ship. Tools such as Expo Atlas or a bundle visualiser show exactly which packages dominate.