In this context, I think “relatability” is connected to plausibility – which, in turn, has to do with the familiar laws of physics. That’s to say, even an impossible physical feat is acceptable as long as it occurs within an otherwise familiar and recognizable reality.
Take an example from
Superman III. Challenged by a massive oil leak from a tanker, Supes uses his super breath to blow the gunk back into the boat. Now to some degree, maybe we can “relate” to and be impressed by the power of “super breath.” Perhaps we compare it to our ability to blow an island of bubbles across the surface of our bath water.
But no matter how hard we try, we wont’ be able to blow those bubbles back into a small hole in our toy boat. So when Supes does the equivalent, we’re more inclined to dismiss the stunt than be impressed by it – because it doesn’t relate to common experience (even allowing for the exaggeration of superpowers).
Make Supes as strong as you like. But that strength is more likely to impress when it’s presented in a relatable context. Without that, Supes’ powers are more like magic (with its ad hoc rules) than they are physical feats.
