That's not universally true (as you seem to indicate already). For example, I included Google's WebRTC libraries in an iOS app and it bloated the binary by several hundred megabytes per architecture. Thinning that would help a lot.

Resources are a big part of it too, of course. A typical iOS app these days has three copies of every image, for 1x, 2x, and 3x resolutions. Dropping that down to one copy on the device is helpful. Of course this doesn't require any of this bitcode stuff, although neither does thinning the executable.

FFmpeg on Android also tends to make apps ridiculously huge when needed due to the need to support multiple architectures. Worse if you want something like x264 as well.

Well-optimized, lean apps often use cache and registers more efficiently. This brings performance boosts. That can save real $$$ on hardware or just let you do more with it.