I think of it as a philosophical difference. In crunchy RPGs the crunch mechanics are an integral part of the character and world descriptions. They're armatures, often hidden from the characters themselves, but still a real part of the characters. And because they're part of the characters' descriptions, it's reasonable for them to be thick and sturdy.
In light RPGs the mechanics are not an integral part of the character, but rather something that gets in the way of describing the "real" character. They're scaffolds rather than armatures, and are treated as a necessary evil due to pure free-form improv not being practical. And because they're considered distracting and obscuring add-ons, it's desirable to make them light and minimal.
Overall, I favor crunch: "Listen to the dice. They speak truths about the game world." Now there are unusual cases where I find the best mechanic to be "GM whim" or even "player decides," but they're just that: Unusual. I mark them as anomalies in my house rules and campaign notes, as places where the usual crunchy mechanics vote "present" rather than either "aye" or "nay."
I think you're onto something but there's one key point I would quibble over. I don't agree that the rules in rules light games are treated as a 'necessary evil'. I think for the most part they are absolutely as essential and valued as in a crunchier game, they are just trying to do a different thing. They're not trying to embody all the different factors that lead in to a particular conflict outcome, like a crunchy game is. But they do still try to be a neutral and decisive arbiter of those same conflicts. They are just 'cutting to the chase' to a certain extent and allowing the various contributing details within the gameworld to be reflected only in the narration and framing or through broad umbrella traits. Note that they may also have other levers available not commonly found in crunchier games, such as escalation mechanics or metagame points. But that 'the system makes the decision' aspect is still absolutely key to making those games work.