Q: Suppose the mass of Earth increased although its size did not, how would this affect the acceleration due to gravity on Earth?

Flexi Says: If the mass of the planet increased, but the size did not, objects on the surface of the planet would experience a greater acceleration due to gravity. Remember that the strength of gravity increases with mass and decreases with distance. If the size of the planet remained the same, the distance of objects on the surface to the center of mass would remain constant. Since mass increases while distance remains the same, the gravitational force on objects at the surface would increase.