Explanations:
Charge density is defined as the ratio of charge of an atom to its volume. Note that the smaller the radius of an atom, the smaller the volume.
According to the formula for the charge density, the smaller the volume of the element, the higher its charge density (The volume being at the denominator).
Hence we can conclude that the high density of Magnessium is related to its atomic radius and shows that the element has small atomic radius.