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.
It is important to remember that carat refers to the weight, rather than the size of a gem.