But building giant fighting robots is a lot more complicated than it sounds
The bigger something is—whether animal or robot—the weaker it becomes proportional to its body weight. [See also: Designers of Giant Robot Hope for Imitators]
That's because when you increase an object's size, what you're actually doing is increasing its length, width and height. Therefore the weight increases by a power of 3 and the energy required to perform increases by a power of 4.