The industry determines the size of a diamond by weight, measured in carats.
One diamond carat weighs 200 milligrams. The industry weighs smaller diamonds using a point system, with every 100 points being equal to 1 carat.
How light strikes the surface, how much enters the diamond and in what form that light returns to your eye — cut dictates all this and more.
Larger diamonds are rarer than smaller diamonds, so all other factors being equal, a single 1-carat stone is worth more than four quarter carat stones put together. However, the worth of a diamond is determined by all of the first four Cs. Size alone does not determine value.