Flutter dialog: how to make scaffold wrap its content?

Question

showDialog(
      context: context,
      barrierDismissible: false,
      builder: (BuildContext context) => Dialog(
        child: Scaffold(
          appBar: AppBar(title: Text('My Title')),
          body: Text('Hello World');
        ),
      ),
    );

The dialog is too big (almost 90% of screen size). Without Scalffold, its size is wrapping content. Can the scalffold be constrained to wrap its content?