You can think of it as a spring is simply a convenient way to bend a metal rod. The more winds you want the longer the rod has to be to make a spring of the same length. If you gripped a rod at both ends with your thumbs pointing to each other, to bend it you want to raise both your thumbs up. It is much easier to do that the longer the rod is. Progressive springs have tight winds at one end so as the spring compresses these collapse onto each other which locks off those winds effectively shortening the rod. This is the same as starting with your hands wide and then moving them closer together on the rod, it will become harder to bend.