Yogg was literally just a head poking out of the ground. The mass of Yogg stretched across the entirety of Northrend. C'thun was described as near death by the time we actually got to him and was only starting to recover. N'zoth by contrast is that big blob that Azshara saw when she sank and has never suffered much in the way of damage.
As for whether he's mobile or not, if he's not mobile that actually gives him an even bigger advantage since he's not just a giant blob, he's also a giant bob at the bottom of the sea, limiting the ways he should be able to be attacked while he plunges his tentacles into Azeroth's tender nether regions. Of course, he won't do this because we are the protagonists, but you catch my drift. In any case I think purely because of gameplay concerns he'll be resized to at most zone-size and at worst avatar-size.
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deenman
Archi is nonsense. Guy was invulnerable to basic everything we threw at him back in WC3 and needed all the wisps to blow themselves to take him out. There's no way that twenty dudes, khadgar and a guy with a really sharp axe would have a chance. Hell, even if all those draenei and frostwolves in the cinematic were involved it'd be stupid. If I wanted to contrive a reason, and I don't, I'd go with the ring carrying the power of the furies (Horde-side) doing what the wisps did in terms of the natural world throwing itself at him.
KJ had top tier NPCs and artifact-wielders against him so I can somewhat buy it. And the implication was always that he was less of a driect combat type than Archimonde. Though nothing will make up for the blue balls of setting up a perfect parallel to Archi taking out Dalaran, alluding to Dalaran crashing in the Khadgar video and then just having KJ gormlessly stare at the projection while sending in more of those papier-mache ships.