Its actually compressed down to a point. A singularity. The 9mm is just the radius from the point from which nothing can return.

For all intents and purposes of astronomic calculations, you can pretend it's 9mm in size, because A) the difference between a 9mm sized object that behaves like a black hole and a singularity with a 9mm event horizon is minimal and B) it's a 9mm black hole going 50km/s+, most math will result in the extinction of human kind anyway.

To be fair, we don't know what is behind those 9mm. Einstein's equations predict a singularity but afaik there is consensus that those don't hold there.