This is not quite what it seems.<p>What happens as you pile mass into a planet is that the planet becomes <i>dense</i>, not <i>large</i>, and this is because of gravity.<p>Jupiter has more than twice the mass of Saturn, but is only moderately larger in diameter.<p>You can keep dumping mass into a planet, and it just won't get much bigger, until you have <i>enough</i> mass that fusion kicks off, and then suddenly the now-a-star inflates, because it becomes extremely hot and then you have something the size of the Sun.