Most of that is probably due to Swift's runtime library.
Surely if you're using core Swift, or core Objective-C, the runtime is a shared lib shipped with the OS? I can't imagine that every single app built on Swift or Objective-C is bundling the exact same foundation? That seems so unlike Apple to miss such a massive optimization to reduce the download size of nearly every app. I can understand there being exceptions with some apps that use custom setups, but surely the majority of apps are written in XCode using exactly what is provided as a toolset.
> I can't imagine that every single app built on Swift or Objective-C is bundling the exact same foundation
In fact, every single app built on Swift is bundling the full set of runtime libraries, both on iOS and Mac. This will continue until the Swift ABI is finalised.
This is why I don't ship Swift yet.
I'm not an expert in the why, but as far as I am aware the whole of Swift is shipped as part of each app as it is still being developed. There is a road map to ship with the OS I believe but it's not for a while
It's because swift doesn't have ABI stability yet.