Perfectly reasonable question, especially since we don’t know the answer. Superfine sugar is simply sugar that has been ground more finely than regular granulated sugar. One of its great benefits is that it dissolves much more quickly in liquids and meringues. It is also called baker’s sugar, bar sugar, berry sugar, and, in Britain, castor sugar. If you don’t have any on hand, you can make your own by grinding granulated sugar for a couple of minutes in a food processor.
Reason would have us believe that, with fewer and smaller gaps around each granule, superfine sugar compacts more than granulated sugar, and therefore weighs more than the same volume of granulated sugar (a cup of superfine would weigh more than a cup of granulated). But, we haven’t found a peep in any reference book backing up that theory. Also, and this is probably the most meaningful point, recipes just aren’t so precise about amounts that you have to worry about it. A pinch more sugar or a pinch less sugar just isn’t going to make a measurable difference in whatever you’re going to cook, unless, perhaps, you’re catering for thousands.