Mirroring won't fill the TV screen. The iPad has an aspect ratio of 4:3 and the iPhone 4S or prior an aspect ratio of 3:2, neither of these will fit exactly into a TV screen which as an aspect ratio of 16:9. The Apple TV will not zoom the mirrored image, because it may well cut off information which is situated at the top or bottom of the screen.
You may wish to try each of the settings in settings > AirPlay > Adjust for Overscan for the best setting that suits you.
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