I don't think it's so much the fact that Ridley is "Too Big" that is the problem here.
What I believe is the issue at heart here, is that the fact that Ridley MIGHT be a boss on Pyrosphere is heavily contested...when it flat out makes sense.
That's not to say that Ridley's popularity doesn't matter, because he's got quite a bit of support. But take the popularity off to the side for a second, and think about what Ridley is. Yes, he's the most iconic antagonist from Metroid. Yes, he's got the ideal unique properties we'd like to see in a newcomer. But at his core, what is he?
He is an iconic, larger than most Boss character. Which makes him ideal for the Boss fights like Yellow Devil showcased in the Direct. I'm not posting this to say that he IS without a doubt a boss, but moreso to point out that it DOES make sense in that train of thought. Ridley is the most iconic villain of Metroid, and is a larger boss character. Just as he appeared as a boss in SSE in Brawl, he makes sense to use as a boss in Smash Wii U, ESPECIALLY because this is a brand new thing Sakurai will likely want to push. Ridley is a front running Boss character. This isn't to say he can't be playable, but to say that he makes complete sense to be a Boss Fight character in this game, like it or not.
Before you say, "What about Bowser?" Or "Bowser confirmed for stage boss". I can explain that.
You see, Bowser...is mother****ing Bowser. Ridley has NOTHING on Bowser.