Gillians seem to be nothing but hulking mindless things. So, perhaps it's a question of the integrity and dominance of the mind. If no single hollow in the congregation overpowers the others, the result is a stupid giant of rather limited abilities. However, since Adjuchas and Vasto Lords seem far more composed and intelligent, it's likely one mind conquered the others and took perfect control over the body, thus also being able to utilize perfectly the raw potential formed by such union.