Why radius of blackhole is so small?

Answer 1

The gravitational (or Schwarzschild) radius of a black hole is relatively small for a couple salient reasons - first, because of the large speed of light, the fastest speed at which things can travel; and secondly, (despite its dominance at large distances), the relative weakness of the gravitational force. If gravity were a more powerful force the gravitational radius of a black hole would be larger; if the speed of light were greater, the radius would be smaller. Another way of stating this would be to consider the radius of the black hole being directly proportional to its mass, but inversely proportional to the square of the speed of light, a large number indeed. If the Earth's mass formed a black hole, it would only be about the size of a marble.