You're right you can't plug a controller to the smoothness
So we have to add it !
Anyway perhaps you can trick this effect by changing the Size with a Controller according to the Speed
OR according to the current stroke Direction : You can make a dot product between a Smoothed 2D direction vector and the current 2D direction, then it will given you a Bend value which can control the size like in your example.
If it's too complicated for you I will try to make a little sample.