Most of the others don't really have anything that lends to them being "Heavier" or "Lighter" by steel standards compared to a variety of mons that don't involve their weight (Aggron is heavy, but what makes it "heavier" as an identity than something like Steelix, Bastiodon, or even Tinkaton swinging its giant Hammer), or just go with both like Duraludon.
I wish it had actually been simpler to understand like "Increase/Decrease Weight Based move damage by 30-50%", as in directly affecting the BP ala Tough Claws. For simplicity sake it could just affect the user's weight (so using Heavy Slam/Heat Crash and being hit by Low Kick/Grass Knot), though I don't think any Light/Heavy Metal users even learn Low Kick.
Some stuff like Metagross and Copperajah get absolutely nuclear STAB moves (plus Heat Crash becomes stronger coverage on Copperajah into its still-current targets), but it benefits them in more match-ups as well as introducing potential downsides (such as Low Kick where Heavy/Light Metal doesn't frequently change the damage bracket now hitting THEM harder). The pool of moves it affects is relatively small at 4 moves (LK, HC, GK, and HS), so I think the effect being easy to track/calculate while being fairly substantial on both fronts is reasonable in exchange. And this is assuming the user even elects to run it over their other abilities (Clear Body plus Scizor's Technician for the Light Metals, Sturdy/Rock Head or Levitate/Heatproof or Sheer Force for the Heavy Metals), with Duraludon (not even Archaludon since it changes abilities on evolving) probably electing for it simply for lack of better options and to weaken opposing Weight hits in Singles (given its special stat bias).
