If the fan was optimized for the duct size, making the duct smaller may actually decrease the current drawn by the motor. If the air is not moving the blades cavitate.
The same can be true of a compressor. When I put the amp clamp on my air compressor I found the peak current was when it had about 90 psi in the tank. It actually dropped quite a bit as the tank approached the 150 psi cutoff.