As I said, black holes only have large masses in nature because that is required to make a star shrink to zero size.

As soon as you get mass with zero size you will have a black hole. This is because gravitational force increases as you approach the mass. If the mass has zero size you can get infinitely close and hence there must be some point around the mass where the gravitational force is strong enough to pull light in.

All black holes decrease their mass. I don't know very much about this but basically it's because of Hawking radiation. As I understand it, in space, particles and antiparticles are constantly being created, they diverge, then come back together, and anihilate. On the event boundary though, when this happens, one of the particles will go outside of the black hole, and its antiparticle will fall back in, and this causes the black hole to gradually loose mass. This would happen very quickly if the black hole had very small mass.