Drop a can of Pepsi and a can of Diet Pepsi into a tub of water. Will they both float? Will they both sink? Will one sink and the other one float?
Perhaps, surprisingly, the diet Pepsi can will float while the Pepsi can will sink.
How can this be since they both are 12 oz? It is because the 12 oz. is a measure of volume, not of weight.
The Pepsi can weighs more because it uses corn syrup for its sweetener, while the Diet Pepsi uses a powdered chemical sweetener, which weighs less.