How can huge and immense objects float on water?

The size of the object is not so much its density, or how much mass it has per unit of volume. A high density means a lot of mass per unit of volume. For example, a rock weighs much more than a piece of Styrofoam the same size because it is much denser. If an object is less dense than water will float, even if it is huge because no matter how much it weighs, the same volume of water would weigh more.