Variable Mix Shapes stretches faces

[_Aka_]

Here is how I would have done things (see PM for the full file)

   

• Instead of 3 separate extrusions, which open the door to misalignment between extrusions if not setup properly, I used only one extrusion.
  

• Instead of using the Variable Mix Shapes, which was intended to mix shapes that are different, I used the Advanced Scaling, since you only need to scale the same shape.
  

• The times of the keys in the Scale Y Multiplier curve are the same as the Relative Distance of the Control Points
  

   

• I set the resolution of the Cross section to a minimal value. Increase it if you use in the future a shape more complex than a rectangle
  
• I set the resolution of the Pah to a fixed value (no optimization), to have enough vertices to ensure a smooth transition between the different scales. It is not much increase in the polygon count, but if you want, you can have 3 extrusions, 2 with optimization, and the 3rd (for the transition) without it. In this case, use the Range setting in each Shape Extrusion model to make it extend only to the right section.
  

I hope this helped.
Have a nice day.