She's cheating a bit, to make a valid point. The gravitational force exerted by an object is due, as you say, to the object's mass-energy. So, a solar mass b

She's cheating a bit, to make a valid point. The gravitational force exerted by an object is due, as you say, to the object's mass-energy. So, a solar mass black hole centered on the center of mass of the sun would have the same gravitational effect on earth as the sun does. But the force a test particle experiences due to the gravity of either object depends on the the square of distance of the test particle from the center of mass of the the object. The physical expanse of the object doesn't really matter, if you're outside of the object. But, with the sun, the closest you can get to the center of mass is roughly 700,000 km. Any closer than that and you're inside the sun. Once you're inside the mass radius of an object, the force you experience is due only to the proportion of the object's mass that is closer than you to the object's center of mass. So the gravitational force you experience (if you could survive being inside the sun) declines as you get closer to the center of mass, until it's zero at the center (the pressure you experience is a different matter - it steadily increases as you journey down the mass). The black hole's radius, though is only about 3km, and so you can approach to within that distance of its center of mass. At that distance from a solar mass, the gravitational force is enormous - sufficient to overcome the momentum of a photon, "drag" it back into the black hole. So, the gravitational force you can experience from an object does depend on it's size, even though the total force at astronomical and mere macro scale distances does not.