
Intuitively, you know that if you increase mass, the force of gravity gets stronger (in other words, when we get fat, we weigh more). If the earth had more mass, you would also weigh more. And if you squeeze a star or planet into a smaller size, the force of gravity also increases. It's the idea of size for a given mass, in other words, the compactness that is important. Why? Because the more compact an object is, the closer you can get to its entire mass. For example, if the earth's mass were compacted into one half of its normal size, you'd weigh 4 times more than you do now! . 
