Thankyou Nugatory. I think that your marvellous formula tells me that the strength of the gravitational force is greater at the surface of the smaller object compared to a larger object with the same mass.
Doesn't that mean that the smaller object has stronger gravity ?
How come the gravity of a stellar- mass black hole is strong enough to trap light but the gravity of a stellar-mass star (eg the sun) is not strong enough to trap light ?