What I mean by game breaking is that they're in the Master Hand category. No one can perform standard attacks that affect the entire stage, or perform cinematic attacks that do extreme damage, unless it is a final smash. For Ridley, these powers are commonplace.
BECAUSE he's a boss. NOT because he could ONLY be designed that way.
You're working backwards from "he's a boss doing boss things" to "he can only be a boss doing boss things."
Never said impossible. I said the odds were slim given all of the factors.
I think his large popularity is a pretty big factor. One that you seem to think is irrelevant. Even though Sakurai has said he is taking that into account and has been the basis for him including several characters that seemingly would not have been in otherwise (Sonic, Mega Man, Villager).
Not covering Bowser again. His size has to do with his power, which varies wildly. He was easily adaptable for Smash.
I'm not talking about whether he's adaptable as a playable character, or his appearances in Mario games.
I'm just pointing out that a lot of the features you're saying make Ridley impossible are
exactly the same features that would be given to many other bosses, even if they have plausible designs that do not involve those. It's a direction of causation thing.
If they made Bowser into a non-playable boss, he would use similar attacks, he would be huge, etc. Because he's a boss. Not because that's the only thing you can do with Bowser. Obviously Bowser is adaptable. I think that Ridley is also adaptable though. Part of the difference with Bowser is that the nature of the series (very light-hearted, cartoonish) makes it much easier for them to put Bowser into a playable role as far as the story and so forth (not that the main Mario games have much of a story anyway).
They wouldn't make Ridley playable in a regular Metroid game because there'd be no plausible story explanation for it, never mind any limitations of the character's physical design. But if they DID, I would guess he'd be considerably smaller.
They DO care about believability. The examples you cited fit organically with the game and for the viewer. Their awareness of that is why they did things the way they did.
And why could Ridley not be a believable character if he was shrunk? This is the crux of what the argument comes down to.
PLEASE SEE EVERYTHING I EVER SAID.
But whether he'd "fit" is subjective.
I still don't see any way you've shown that 1. that's objectively true (which is impossible to show anyway) 2. that the majority or even a significant plurality of fans would dislike it 3. that the game creators would be so rigidly concerned for Ridley's canon size or 4. that he doesn't have move set potential.
If the fans would like it, if he has a potential move set, if they have a cool design... I can't see them rejecting it on the basis of this version of Ridley would be too small and thus not consistent with the Metroid games.
Do you really think he'd choose disappointing the fans over violating ONE aspect of canon?
You must refer to EVERYTHING HE'S EVER POSTED as he says. Shall I get you a snorkel as you go swimming through them? Or would you prefer a clothespin for your nose? 
