Windows Phone 8 wrong backbuffer size

I create project from Windows Phone 8.0 template. When i deploy my app(that should shows only blue screen), backbuffer size is wrong (is to small). When i changed Grid to DrawingSurfaceBackgroundGrid, backbuffer size is ok, but my project is not landscape - is portrait :confused: How can I fix it?

I use lumia 635 with windows mobile 10