The two main ways of reducing the size of the player are by making proper Release build within Xcode and by changing the Stripping Level within Unity.
It is expected that final release builds are made using Xcode 4.x/5.x command Product -> Archive. Using this command ensures that build is made with release configuration and all the debug symbols are stripped. After issuing this command latest Xcode switches to Organizer window Archives tab (you can navigate there manually via Window -> Organizer menu). You will find there two very useful functions there: App Store size estimation and Distribution. Build size estimation function works pretty well, but it is always recommended to have some small extra margin for error when aiming for 3G download limit (which currently is 100MB).
The size optimizations activated by stripping work in the following way:
Strip assemblies level: the scripts’ bytecode is analyzed so that classes and methods that are not referenced from the scripts can be removed from the DLLs and thereby excluded from the AOT compilation phase. This optimization reduces the size of the main binary and accompanying DLLs and is safe as long as no reflection is used.
Strip ByteCode level: any .NET DLLs (stored in the Data folder) are stripped down to metadata only. This is possible because all the code is already precompiled during the AOT phase and linked into the main binary.
Use micro mscorlib level: a special, smaller version of mscorlib is used. Some components are removed from this library, for example, Security, Reflection.Emit, Remoting, non Gregorian calendars, etc. Also, interdependencies between internal components are minimized. This optimization reduces the main binary and mscorlib.dll size but it is not compatible with some System and System.Xml assembly classes, so use it with care.
These levels are cumulative, so level 3 optimization implicitly includes levels 2 and 1, while level 2 optimization includes level 1.
Note that Micro mscorlib is a heavily stripped-down version of the core library. Only those items that are required by the Mono runtime in Unity remain. Best practice for using micro mscorlib is not to use any classes or other features of .NET that are not required by your application. GUIDs are a good example of something you could omit; they can easily be replaced with custom made pseudo GUIDs and doing this would result in better performance and app size.
Refer to documentaiton on managed bytecode stripping with IL2CPP for more information
Note: it can sometimes be difficult to determine which classes are getting stripped in error even though the application requires them. You can often get useful information about this by running the stripped application on the simulator and checking the Xcode console for error messages.
An empty project would take less than 22 MB in the App Store if all the size optimizations were turned off. With code stripping, the empty scene with just the main camera can be reduced to less than 12 MB in the App Store (zipped and DRM attached).
When publishing your app, Apple App Store service first encrypts the binary file and then compresses it via zip. Encryption increases ’‘randomness’ of the code segment and thus makes it worse for compression. Check “Building for distribution” chapter above how to estimate App Store size before submission.