I always thought of the Colossi as representing aspects of Dormin, with their size corresponding to how strong those aspects were; the tiny Colossi represent impulses or passions, which a creature like Dormin would barely have, while Phalanx, which never even attacks you, could be seen as contemplation or wisdom. The last, the towering, nightmarish figure of Malus, literally smolders with hate; your final battle therefore with the very personification of Dormin's anger at those tho did this to him.