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The dynamic libraries in iOS aren't shipped as separate dylibs on the device. Rather, they're essentially concatenated together as part of a single file called the dyld shared cache that also lets Apple do some prebinding tricks, interning of objc selector names across all system libraries, etc.

The system-wide iOS build scripts actually randomize the order of the libraries in the shared cache by default (you can check this using dyld_shared_cache_util against the shared cache file, which you can compile from the dyld project on Apple's open source site). Since the ordering of dylibs in this giant shared cache file varies between builds, you could easily end up with a 15 MB diff even though all you've done is deleted a single goto in source code.

The update probably includes the actual updater executable, not just the diff. This would allow you ship different kind of patches (from simple fixes to entire OS upgrades) using the same mechanism.

they also bundled other NSA holes with this fix. :)

seriously, tho, it is probably the whole binary this was part of.