The size of the set has been made to fit regular human size for the shoot but on screen you can see that actual humans will be tiny. Clearly not as tall as Fassbender is in the middle of the plateform...
So the human characters will be reduced in post to make the look Engineers huge.
Easier that the other way around i guess.
As for the mutated guy, its settled.
No human can jump like they do, especially outside of the ship without helmets ^^