Spiraling staircase for lighthouse

I am trying to to create a lighthouse with a spiraling staircase. Building the spiraling staircase is easy. The part I am struggling with is having the inner radius of the staircase be smaller as it spirals upwards. The radius of lighthouse decreases the higher up you go. Anyone know how to change an existing spiraling staircase (or even a regular cone) where the floor radius is larger then the ceiling radius?

 

Maybe use a taper tool either in your modeling tool or in Unity something like the Mega-Fiers?