The correct statement is:
Terrestrial planets are more dense, in part because they have so much more mass.
Terrestrial planets (like Earth, Mars, Venus, and Mercury) are composed primarily of rock and metal, which makes them denser than gas giants (like Jupiter and Saturn), which are primarily composed of lighter gases and ices. While mass does influence gravitational effects and the structure of a planet, density is more closely related to composition rather than size or mass alone.