AVCaptureVideoPreviewLayer看起来像iPhone 4大小

我正在使用以下代码在我的iOS 9，Swift应用中显示相机：

特性：

@IBOutlet weak var previewView: UIView!

var session: AVCaptureSession!
var input: AVCaptureDeviceInput!
var output: AVCaptureStillImageOutput!
var previewLayer: AVCaptureVideoPreviewLayer?

委托方法：

override func viewDidLoad() {
    super.viewDidLoad()
}

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated)
    setupSession()
}

override func viewDidAppear(animated: Bool) {
    super.viewDidAppear(animated)
    // Experimental / debugging
    view.layoutSubviews()
}

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    // Experimental / debugging
    previewLayer!.frame = previewView.bounds
}

拍照方式：

func setupSession() {
    session = AVCaptureSession()
    session.sessionPreset = AVCaptureSessionPresetPhoto

    let camera = frontCamera()

    do { input = try AVCaptureDeviceInput(device: camera) } catch { return }

    output = AVCaptureStillImageOutput()
    output.outputSettings = [AVVideoCodecKey: AVVideoCodecJPEG]

    guard session.canAddInput(input) && session.canAddOutput(output) else { return }

    session.addInput(input)
    session.addOutput(output)

    previewLayer = AVCaptureVideoPreviewLayer(session: session)
    previewLayer!.videoGravity = AVLayerVideoGravityResizeAspect
    previewLayer!.frame = previewView.bounds
    previewLayer!.connection?.videoOrientation = .Portrait

    previewView.layer.addSublayer(previewLayer!)

    session.startRunning()
}

func frontCamera() -> AVCaptureDevice! {
    let devices = AVCaptureDevice.devicesWithMediaType(AVMediaTypeVideo)
    for device in devices {
        if (device.position == AVCaptureDevicePosition.Front) {
            return device as! AVCaptureDevice
        }
    }
    return nil
}

但是，相机显示如下：

它似乎等同于iPhone 4的显示尺寸。

我该如何解决？我尝试将框架设置为viewDidLayoutSubviews，但无法正确调整其大小。