I\'ve been going backwards in Git history to find the cause of a humongous file size change, but the only real reason I can find is the switch from Xcode 6 to Xcode 7 GM.
Assuming you don't want to disable ENABLE_BITCODE, you can strip the debug symbols.
See http://oguzbastemur.blogspot.com/2015/09/xcode-7-and-increaded-binary-size.html as stripping debug symbols away before embedding bitcode is one of the options you can do.