Flutter: Print image with right orientation and size A4

I want to print an image in flutter. I use the package "printing 3.5.0". My problem is now I want use the hole space of the page. But when I put the orientation or size, both parameters are ignored.

_printImage(GlobalKey globalKey) async {
   final pdf = new PdfDocument();
   final page = new PdfPage(pdf, pageFormat: PdfPageFormat.a4);
   final doc = pw.Document();
   final g = page.getGraphics();

   RenderRepaintBoundary boundary =
       globalKey.currentContext.findRenderObject();
   ui.Image _image = await boundary.toImage();
   var bytes = await _image.toByteData(format: ui.ImageByteFormat.rawRgba);

   PdfImage image = new PdfImage(pdf,
       image: bytes.buffer.asUint8List(),
       width: _image.width,
       height: _image.height);

   doc.addPage(pw.Page(build: (pw.Context context) {
      return pw.Center(
       child: pw.Image(image),
     ); // Center
   }));
   g.drawImage(image, 20.0, 0.0);

   Printing.layoutPdf(onLayout: (pageFormat) {
      return pdf.save();
   });
}

Even if I adjust the width and height of the image, the whole page is not used. Does not exist a parameter that automatically scales the image on the whole page and observes the orientation?

I'm grateful for any help.