The springs are created with the right resistance from the factory at the size that they are made. For example, a spring that is half the height of stock can be created, through thicker and stronger materials, to ride smiliarly and have a similar resistance rate to a full size spring. When you start cutting springs, you need to accomodate the thickness of the spring so they will not bottom out. You will lose handling altogether, and you will bottom out at every bump in the road.
Let's add that they are progressive and not linear wound.
Cutting the spring changes the characteristic and basically ruins the spring performance.