Here’s a picture of one of my cars with progressive springs. I put the word in quotes in an attempt to signal that it is used loosely - same with frequency. I agree that these words a not always used technically correctly, and I am guilty of that, but at this point it’s RC jargon that people can intuit as let’s say “dynamic behavior” of t We should expect the frequency of the longer spring to be lower assuming that it has more mass. Again I think the most important factor is preload