Unless you're importing a ridiculous amount of packages and assets, you'd never hit 100MB with simple apps, that's just people looking at debug mode and thinking that it's the final app size. Simple apps can easily be under 8MB once compiled.

Not really true, you are likely comparing a debug build. An Android Flutter apk sits around 4MB for smallest app. Most of my apk sizes are around 8MB-10MB w/ Flutter

I haven't written any myself, going from reports online that builds are a lot larger than expected. That minimum size is for an app without any components or other modules. The RN app I mentioned was ~6MB with a fully blown app, state management, heavy data layer, hundreds of components, animation, i18n, a handful of native modules, encryption, and so on.

In short, Flutter doesn't really have any advantage in size over RN.