I think the Fierce Diety is a physical amalgamation of anger, and Link probably has *some* latent anger/negativity over things (we've had this discussion... sorta... about my Link headcannon, I think) and the mask made all of that surface- Link was always capable of that great, dark power, but the curse or container of a physical body (once a child/adult's body, once a scrub's) inhibited it. FD isn't really a *positive* Diety, after all, so the mask kind of... unleashed the monster, so to speak, and shed the presumptuous foolishness of someone who denied their hatred and bitterness and embraced it- constructively, of course, because the moon (presumably) does not fall.
The MM Link relies on false appearances, and that includes his wee child body- his "true" form (is it his true form? He says nothing and he has so many false faces, so we can't really know, after all... it's all interperetation) is the ultimate disguise!
That's more or less what I had. I figured that, given what he goes through, Link is pretty much one of those larger-than-life heroes from mythology or tall tales. Taking down giants? A dark lord? Even a demon? Link is capable of a whole lot of destruction, but maybe all people see is the face of an innocent child, or perhaps even the encouraging face of a hero. But that's not what his enemies are going to see. For them, there is no mercy!
Or something. I actually just came up with that last part, with my main idea originally playing around with Link's potential for breaking things, be it pent-up angst finally being released or righteous fury.
The thought of Link being a destructive god is actually really funny, given the whole Demise/Hylia thing. PERHAPS GANONDORF WASN'T HIS REINCARNATION AFTER ALL!!!!!!!! *eyebrow waggle*
But the idea of Link being ready to BUST feeds into some real "it's not easy, being green" jokes except in terms of the Hulk rather than Kermit the frog. Link SMASH!!!!! (although the Fierce Deity smashing Majora and ending with a "Puny god" is pretty accurate as far as @$$-kickin' goes, so maybe?????)
So I'm not to sure if your implying that Link(Oot MM) is really the fierce deity trapped in human body who's powers got sealed away in a mask. Or that the fierce deity managed to take over Links body, reducing what is left of Link into a mask with all his powers and abilities. Well either way, its a pretty good piece.