Yes, miso uses the JS backend in GHC, and mobile phones have embedded JS interpreters (e.g. JavaScript Core). These interpreters can access native libraries to draw native views, or access native device APIs.
Projects like lynx and react-native automate this process using something akin to node-gyp, exposing kotlin / swift libraries via C ABI w/ a JS API. Miso accesses the kotlin / swift native modules by FFI'ing into the JS that exposes them.
The JS doesn't get compiled, but on Android it does get JIT'd. So it's "native" in the sense that the views drawn are native (not WebViews), and the device APIs are native, but not "native" in the sense that it's compiled.
There is a company that has a 400 module miso application and the bundle size is not an issue (https://www.polimorphic.com). After closure compilation GZIP'ing, pre-rendering and caching of the JS, it's a one-time cost, and is negligible due to pre-rendering. We build websites for users, who care only about experience, not about how large the payload is. Payload only matters insomuch as it adversely affects user experience. This argument is strictly a developer concern and is in no way correlated with end-user feedback. There are many <1MB js payload size websites with 0 users.
This post dismisses Elm on the basis of lack of typeclasses, but fails to mention miso (https://haskell-miso.org), that includes typeclasses. Miso is Elm-as-an-embedded-DSL written as a Haskell library for GHCJS that allows one to share types on the front and backend, take advantage of the entire Haskell ecosystem and has experienced commercial use.
If decoding json in Elm is considered hard, I'd recommend checking out miso (https://github.com/dmjio/miso), a Haskell re-implementation of the Elm arch. It has access to mature json libraries like aeson for that sort of thing, along with mature lens libraries for updating your model. Here's an example of decoding json with GHC.Generics using typeclasses.
https://github.com/dmjio/miso/blob/master/examples/xhr/Main....
If you want a powerful type system (i.e. Haskell), but the benefits of Elm, Miso is a project that implements the Elm Architecture in Haskell. It obviously has typeclasses, and can encode / decode JSON on the frontend using GHC.Generics quite well. https://github.com/dmjio/miso
Why thank you kind sir :) Using advanced optimizations with google's closure-compiler, the todo-mvc example can get down to 500kb. Compressed it might even be smaller. This is why having the "isomorphic" part is important. This way the user will see content immediately while the app bootstraps in the background and sets up event delegation. Then events will get routed through the vdom to pure haskell functions and the app is responsive again.