Resize 2 parts of object with different ratio .

Hi all :)

I've an insole corresponding to a certain shoe size.
I need to resize this insole in order to have the right width for another shoe size.



The problem is that I've to resize (in x) the front and the back independently



For a specific case I need to increase front width by 12mm and back width by 4mm. But I need to keep an harmonious mesh.

Here's my blend file: http://pasteall.org/blend/index.php?id=47428

Do you have any idea? 
Thanks a lot

Maxime
Select vertices on both sides of the mesh, press S > X > LeftClick. Then you can tweak settings in Tool Shelf (left side of the window - T) or by pressing F6.