Why is viewWillTransitionToSize the wrong size?

I am adjusting the limit in the iOS8 rotation delegation method, but the size is not the same as what I can see on the device, why?

override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {

    println("size: \(size)")
    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
    widthConstraint.constant = (size.width / 8.0) - 4.0
}

      

        
        
        
      

    

size: (1024.0,768.0)

But the actual size is smaller than the full size of the iPad.

Testing on a real device and viewController is a built-in container view controller.

I seem to be getting the size of the parentViewController view somehow.