I don't know where crossjunkie is coming from to be honest. Length of cage is dictated by how much slack in the chain it has to take up. If you're running a triple chainset, you need a chin long enough to wrap round 46T at the front and 30T at the back, but that's going to flap about if you're on 22 at the front and 11 at the back, so you need a sprung lever arm (cage) that's long enought to take upthe slack on small small,
The bigger the difference between potential gear combo's, the longer arm (cage) you need to move far enough to keep the chain taut on the smallest/smallest combo and still be able to wrap round biggest/biggest.
How strong the spring is is independent. I'd assume they use duifferent strenght springs, but maybe they don't clutch mech makes this irrelevant anyway.
If you're on a 1x_ set up, you only need a short cage, because the difference is small - no big changes at the front.
If you think you might be changing front rings (eg. 48T for commuting and road rides, 38T for off road rides, like I do) a medium cage means you don't have change the chain length, because the cage is long enough to take up the slack on the smaller ring.