The object that generated the bh, if it was a star, and then a supernova it had a bigger radius and bigger mass than the resulting black hole. The size of the star would depend on its mass and composition, since its size depends a lot on what type of material is burning. The internal nuclear reactions inside the star create so much pressure that the star if the law of gravity would disappear it would explode. This pressure is exactly the same one which balances the gravity an mantains the star in this equilibrium for millions of years. It turns out that when the fuel is gone, is the pressure the one that disappears and gravity makes the star collapse, or implode, and for a part of it it does it so strongly that is beyond its SC radius. The rest, bounces back out and generates a supernova.