When I build for the iPhone in Unity the filesize is huge compared to other target platforms. Any ideas why and how I can fix it?
at 13mb I would expect an iOs build size of around 40mb. Are you building a Universal arm6+arm7 version, because that could effectively include two versions.
I too was absolutely shocked at the size of the iPhone project. In fact my Web build was 7mb including all assets, but an empty iPhone project is 17mb with no assets. This is just how large the core Unity engine is.
To help, Minimise your file sizes- that goes without saying. Also consider doing individual builds for arm6 and arm7. Also, try not to use the system library (I think but I am not 100% sure, that many string manipulation functions use this library, although it’s not the only thing that does.) the system library uses 1mb or so.
Also, compress your textures. They may not be compressing as effectively on iOS.