nope. You are not going to be able to do this by either scaling the smaller one up or the large one down. When you scale EVERYTHING is going to stretch or get compressed.

One way to do this is to create a profile and two curves (an inner and outer curve) and then extrude the profile along the curves. The size of the section created by the profiles will be the same.

You'll have to model the notches in afterwards.

and done.