What scale is the map you are creating the radius on? Is there a similar relationship of the size of circle you are drawing to the circle being mapped? What is the native coordinates system for the data you are drawing the radius on?
Sometime when looking at this sort of problem it helps to step back, look at the whole problem and see if you can isolate individual components that may be producing the undesirable outcome. Once you isolate them you can then systematically research for solutions, or set up experiments that will systematically work through one variable at a time until you get to a solution. Of course the more you become familiar with the tools of your trade, this experimenting process becomes less formal and more 'intuitive', but you have to start developing a method for solving these sorts of questions so when a larger more complex problem occurs you are able to apply the tried and trued 'solution developing methods' that you have built up over time.
HTH,
Mark Shymanski
