I asked that question at the worldbuilding stack exchange.
Obviously if a small world had a primordial black hole with a greater mass inside, it would have a higher surface gravity and escape velocity than a world of that size made out of even the densest known materials. Thus the possibility of a fictional habitable world much smaller than otherwise would be the case.
The answers to this question What would happen if a small black hole fell into a star? indicate that I might get a good answer here.