Take apart a ball point pen, one of the click types.. There is a spring in there. Usually linear. If you pull it so as to stretch the coils on say half of it, then compress it you'll find the close coils the ones that were as original, will all compress before the ones with the bigger coil spacing. You just made a progressive spring.
The number of coils in a given space changes rate even if you don't change wire diameter, or coil diameter of a spring. Why? Well I'll let an engineer attempt to explain that...
To make matters worse, there are progressive springs that gain rate all the way through their travel, and progressive springs that are more appropriately called dual-rate where there is one rate, that transitions right to another. Most "progressive" automotive spring are that way, some are not. And there are times a spring like that is required since the stiffer and shorter a spring becomes if you don't have what amounts to a built in tender spring, it'd come loose at droop.