It's called centre-piece figures. It's not that Predaking, Devastator or Omega Supreme are that much larger, it's because they're a large and visible marketing figures. Kids that aren't into Transformers will see them and may be interested because BIG ROBOTS is cool.

Just move on the scale thing, HasTak know it's not in scale and never had any intention of making them scale. Just like how movie Bumblebee has so many Leader class toys even though none of them scale.