It is more complicated than that, though. Yes, while you can build a kernel+libc+binary code application that is pretty lean, that means that your kernel plus

It is more complicated than that, though. Yes, while you can build a kernel+libc+binary code application that is pretty lean, that means that your kernel plus graphics stack need to handle both that type of application, as well as a web browser and related content. In some cases, having to handle just one of those opens up optimization opportunities, either in the kernel, libc, or the graphics stack. If you can run only native binaries using a single graphics method, that might be overall leanest, but every phone needs a browser these days. So the only other option to have a single thing to optimize for is to make the only stack the web stack. Then, whatever is optimal for the browser in terms of handling GPU textures, as one example, you can just do, because nothing else needs to run as well.