Ridley had a more bulkier build in Other M.
So no, it's not the fact he's "skinny yet giant" that's the issue.
Nor is it the idea that he's not "intimidating" at a smaller size.
Because BaganSmashBros. old version of his Ridley mod for Brawl is
plenty intimidating.
The problem with Ridley (namely the Other M model since that would have been the Ridley focused on for Smash 4) that BaganSmashBros.' mod
unintentionally draws attention to is that the design requires
several noticeable and drastic tweaks in order to even remotely function on a basic level as a playable fighter. (And even then, there's still something....
off about how it functions.)
To the point it isn't reflective of the source material, which is what Sakurai wanted to avoid.
I'm not saying the mod doesn't show Ridley can be made into a fighter, but when I look at
that Ridley, I don't see Ridley (as in, who he is in the Metroid series). I see some new creature that kinda
resembles Ridley (doesn't help that there were some major aesthetic tweaks that weren't for gameplay purposes on top of it).
The current mod of Bagan's that uses the Brawl design however, looks more "natural", even at the smaller size than the boss.