I think one of the leakers (Josh?) said that some little boy NPC mentioned seeing a giant gourgeist [but no one believing him] so Giant Size probably exists.The Gourgeist shown in this thread is apparently its "small form." Some have suggested that there could be different forms based on its weight. Perhaps to key to finding out is either that or the move Growth (common move among grass types).
That said, I wonder kind of effects its forms may have if there are different effects at all. I think there has to be some kind of gimmick as it'd be entirely redundant for Gamefreaks to create two grass/ghost pokemon giving one an ability to differentiate itself from the other.
Since it's not referred to as "Form" I do wonder if it might have distinct stat differences....which would only really be helpful if anyone could actually find how to evolve the thing in the first place
Worst case scenario: The reason it got Huge Power was to offset losing Attack so it could get more stats elsewhereIf this turns out to be correct - and the boosts seem fairly accurate - let's hope Mawile doesn't get a boost outside of its defenses, from 50/85/55 to something along the lines of 50/125/115 would be as nice as it gets.