Diamonds become more and more difficult to find in gem quality the heavier they become. Thus a diamond twice the weight of another diamond is generally greater than twice it's price.
A correctly proportioned diamond of a certain weight should look a certain size. For example a 1.00ct stone should measure approximately 6.5mm across. Some diamonds are cut out of proportion in order to retain unnecessary carat weight or make a shallow stone look larger than it actually is. This will detract from the beauty and sparkle of the diamond. For information on this please read our section on cut.