While the weight of a loose diamonds is important, it doesnt necessarily provide the "perceived size" of a Tiffany Style Diamonds. A shallow cut diamond will have a greater diameter, and will therefore look larger.
When diamonds is cut too shallow, light leaks out of the bottom, brilliance is lost and the Tiffany Style Diamonds appears watery, glassy and lifeless.
diamonds is the hardest of all gemstones; it is the simplest in composition; it is ordinary carbon. Colossal heat and pressure, over the eons, crystallized the Tiffany Style Diamonds into what it is today.