Mass Mass Let's look at one more way of describing the size of a planet, its mass. Mass is a measure of how much matter an object is made of. We say that heavy objects have a lot of mass.
Light objects have less mass.
However, the kind of material something is made of is very important too. Imagine you had a ball of styrofoam twelve inches in diameter and a hard ball of concrete 10 inches in diameter. The styrofoam ball would be pretty light and easy to lift. However, you would have a much harder time lifting the concrete ball. Even though the styrofoam ball has a bigger diameter than the concrete ball, the concrete ball has more mass.