React Native JavaScript bundles on iOS and Android are usually significantly larger than app bundles on the web because the runtime and core components result in a ~700kb baseline size. This is not as big of a concern as on the web because JavaScript bundles are typically not downloaded at the time an app is opened, as you would on a website, rather they are embedded in the app binary or downloaded in the background with expo-updates.
Large bundles can impact startup time because they need to be parsed and executed when the app launches, and this can be slow on low-end devices. You can improve startup time on low-end Android devices in the bare workflow by opting in to using Hermes and precompiling your app to Hermes bytecode at buildtime.
You can analyze the contents of your JavaScript bundles using react-native-bundle-visualizer. It will show you how much each package in your project contributes to the overall bundle size.