Density is a physical property of matter that is expressed in mass per unit volume, typically in grams per milliliter, and signifies the compactness of an object. Mass is a measure of the quantity of matter, and differs from weight in that weight is the force exerted on a body by gravitational attraction. Low density, therefore, simply means that a body possesses a low concentration of mass.
Size May Not Matter
To illustrate the difference between high density and low density, consider what would happen to a beach ball and a bowling ball if both were dropped into a container of water large enough to completely submerge each ball. The beach ball would float on the water's surface, because the exterior is composed of lightweight plastic and the interior is occupied by air - the latter of which is less dense than water. Conversely, the bowling ball would splash down to the bottom of the container, because it is composed of multiple layers of tightly compacted materials that are more dense than water.