Check the curve radius property.
Under edit mode each vertex has a Radius value that acts as a scaling factor for the bevel width. If the bevel width is 9 unit, and bevel 0.5 then you will end up with a bevel of 4.5 units. It will also affect the scale of bevel objects.
You can control per Vertex Radius from the Properties Shelf in Edit Mode under the coordinates boxes or with Alt+S for proportionally changing the values. You can also override all selected vertex at once and set an absolute value with the Set Curve Radius Operator (Space Bar menu).
Also check the object scale in Object Mode and make sure it is 1, if not apply it with Ctrl+A > Scale then and afterwards go through the steps described above, since applying uniform scales to curve objects also affects per vertex radius values.