1.) You're just wrong, my dude. It absolutely requires that the only thing that changes is proportion of the object. Otherwise, you're not doing a comparison of a scale up object, you're looking at two completely different objects.The square cube law refers to the shape of an object and its materials, not being the literal exact same object.
2.) Materials being the operative word here. Again, I don't need to assume that a giant is made of the same materials as a human.