If you are playing a 16:9 video or TV program it will use the full screen. If you are simply mirroring what's on the iPad, say an app or a web browser window then you get the same aspect ratio as the iPad. If you converted it to 16:9 it would look strange everything would be stretched and look short and fat.
You can change the aspect ratio on the iPad for the video size. Then when mirroring, it will fill your screen. Take the 16.9 off the iPad by clicking the icon in the upper right corner.
You can change the aspect ratio on the iPad for the video size. Then when mirroring, it will fill your screen. Take the 16.9 off the iPad by clicking the icon in the upper right corner.