Big Dude wouldn't last 5 minutes in the cage with an MMA pro. Just look at Mariusz Pudzianowski who actually tapped to strikes. Submitting to strikes is considered the biggest pussy act in MMA and yet the strongest man in the world did it. You nutters need to get it out of your head that size equals fighting ability. No matter how big a man gets his head remains just as vulnerable and feels the pain the same way when it is punched repeatedly.
I don't think ANYONE thinks this. But in the way a train engine doesn't possess any fighting ability but will still destroy much of anything in its path, so, too, will an individual who is vastly superior in size and strength to an average Joe.
To be so monstrous and strong and powerful is to have a considerable advantage in a physical altercation. As with cardio, flexibility, co-ordination, etc., strength and power are physical attributes of huge worth in any hand-to-hand battle.
Even from a purely defensive standpoint, massive size and strength provides armour and structural resilience.
And your suggestion about the head, throat, etc. being every bit as vulnerable? You're correct. However, to strike any of these areas in a one-shot-one-"kill" manner is not all that easy a task. Especially when someone of ungodly force capability is attacking with aggression and intent.
P.S. Your Mariusz "example" is deeply flawed. Fact is, for a man of his almost-zero-cage-fighting experience, he was able to hold his own for a time against an elite-calibre former world champion fighter (who is also
substantially taller than he is). You're reading his relative failures as a MMA fighter in reverse.