XXX,
In principle the single approach point SHOULD be a small area but by the time you factor in the people that call inbound:
- 2 miles away "approaching 2RN"
-1 mile away "approaching 2RN"
- overhead "2RN"
- up to 2 miles passed 2RN becuase the frequency was congested and they couldnt get the call in so they call "just passed 2RN"
and the fact that the aircraft is travelling anywhere from 80 - 140+ kt as they are making the call you have an area that its at a guestimate nearly as wide and as long as a circuit and so the area where traffic arrives is of a similar size.