See those little birds out there in the sky? They're there to show that Sphinx of Uthuun is bigger than them. The presence of tiny "scale birds" shows that the main creature is giant.
Gaea's Revenge is so big that those birds probably just think it's a mountain or something.
Without birds: a guy with a sword. With birds: a titan with a sword.
Technically, the art for Primalcrux has scale bats, not birds. But that's close enough, right? Wayne Reynolds is a master; he can do that.
That's actually not the original version of Hedron Rover. During development, the creature was a bit larger, so the first illustration looked like this:
See? Change the scale birds and you change the size of the creature!