Don't be so hard on yourself. You are just confusing Mass with Size. The first is a measure of density, the other a measure of distance.
Mass v. density, what has more mass, a ton of feathers or a ton of lead?
Mass is emphatically not a measure of density. It is a measure of mass. Density is (usually) a measure of mass per unit volume, but can also refer to obstinate or ignorant thinkers.