Not home now but I believe ATV's default overscan setting (if there is an overscan setting) is 1:1 without any overscan. If there is an overscan setting just set it to 0.
Overscan settings are usually only in the monitor (your HDTV) and not the source. Is your TV set to 1:1 pixel mapping with no overscan.
The Apple TV is putting out by default a 1080p resolution which is scaled back for 720p and older TVs.
You are looking for a button like "HDMI Zoom". If you report back with what TV you have I can tell you what button will do what you want. This is not on the Apple TV's end though.