The size of the Wwise Unity Integration packages vary depending on the type of library (Debug, Profile or Release) and the target platform.
The Debug versions are the biggest because they include all debugging symbols, these are useful when investigating application crashes during development. The Profile versions contains the code used for the communication between the Wwise sound engine integrated in the application and the Wwise Authoring Tool's Advanced Profiler; useful when investigating audio issues in a running game. The Release versions are the smallest as they don't include debugging symbols or profiling functionality, these are used in the final version of the game.
When creating a Unity project targeting iOS; Unity will create an XCode project that will link with Wwise libraries. The libraries contain all debug symbols *even* in Release. Also, they are source libraries that haven't gone through a linker yet; some optimization can only be done at link time. You need to specify the proper stripping option for the linker to reduce the size. Please refer to Unity documentation for more information.