Use a bevel object. You can enter the name in the Bevel Object field in the Geometry panel of your main curve object.
A bevel object is a second curve object that is extruded along the main curve. This gives you great flexibility to choose how the cross section of your main object should look like since you are not limited to rounding the edges. In that example however, I just imitated the original bevel.
I set the bevel bezier curve to Vector with the V menu. That produces straight lines. I also put the object origin of the bevel curve on the curve. The origin of the bevelling curve is the reference point for the bevelled main curve.
In the picture below, you can see(from left to right):
- The ordinary bevel
- The larger diameter, caused by an ordinary bevel.
- The bevel object with it's object origin in the right place.
- The resulting bevelled curve object with exactly the same diameter as the unbevelled curve.
