Xcode Archive Is Too Large
Have you read this?http://docs.unity3d.com/Manual/iphone-playerSizeOptimization.html
Also it can be fault of stripping level, try to use micro mscorlib.
In addition you can take a look at Decrease Your App’s Code Size from Mac App Programming Guide.https://developer.apple.com/library/mac/documentation/General/Conceptual/MOSXAppProgrammingGuide/Performance/Performance.html#//apple_ref/doc/uid/TP40010543-CH9-SW2
I believe part of what is happening here is the additional overhead for the arm64 slice (unless you were already including the arm64 slice previously). I would not expect that much of an increase in size for the arm64, but it would still be roughly X2 on the app binary (minus the resources).
As of Feb 1, Apple requires arm64 support as well as the app being built with the iOS 8 SDK. The default Xcode build setting enables arm64.
What you can do is Show Package Contents on the xarchive and work your way to the app executable binary. You can then compare the size difference on the app executable binary from before and now. You can also run lipo -info on it to see all the slices from before and now. Note I am assuming you have an older xarchive to compare with.
I would then probably diff the rest of the resources (use something like Araxis merge) to see the differences in the files. This will let you see what resource files changed or got added. If your diff is only the executable, then you have isolated were the size difference has come from.