Leaf springs are commonly called elliptical or semi-elliptical,as they are shaped like a section from an ellipse. Held at both ends and loaded in the center, when the three points are in line the leaf spring which started out as an elliptical section will be flat and straight, which is the desired end condition, which is why they are shaped like an ellipse to begin with.

The typical vintage design leaf spring has a leaf or leaves that are made from flat strip stock, equal thickness and equal width for the full length. When a leaf like this is bent by opposing force at three points, it sees highest stress at the center point and lowest stress at the end points. Low stress metal in a spring means extra weight required for no useful purpose, as the whole spring has to be thick enough in section to withstand the highest stress at the center point with the desired load.

Next step is to stack up multiple leaves that are progressively shorter. This effectively places more material near the middle of the spring and less material near the ends. This reduces stress in the middle and increases stress near the ends. The idea is to have more equal stress over the length of the spring so you don't have excess material at the ends, and therefore a lighter spring.