The width of the ductwork centerline corresponds to the elevation of the ductwork. It is intended to give you some feedback about the relative height of your ducts when viewing them from above.
If the centerline appears to be too wide, check your DIMSCALE setting. To do this, type DIMSCALE at the command line. If it is 1, it is definitely wrong. If you are unsure whether it is correct, you can learn more about setting DIMSCALE.