A large empty object has a lighter visual weight and so it can be balanced with a smaller filled, or more dense object. For instance a blog with a content area and side bar has two distinct sizes of objects, the content area is usually larger than the side bar. You could balance the two by adding more white space the content area and filling the side bar with something more visually heavy. If you want to accomplish symmetrical balance in your design then you’ll need to make sure elements are the same size, regardless of visual weight.