If you zoom HD CCTV cameras in so they cover the same area as a standard definition camera you will get a clearer image. Pixel density increases and the image improves.
If you maintain the same pixel density as standard definition then an HD camera will have a longer range and be able to film a wider area. In crude terms a 1080P camera will be able to capture the same detail as a standard resolution camera at twice the distance.
With standard definition equipment you can film about 3.5 metres wide and identify someone. This increase to about 8 metres when using 1080P cameras. The reason the increase is more than twice is because HD 1080P records in 16:9 aspect widescreen and standard D1 resolution records in 4:3 aspect ratio.
The following images show improvements achieved with the HD camera zoomed in to cover roughly the same area as a standard definition camera.
HD 1080P image (click to enlarge).
Standard D1 resolution (click to enlarge)