Suppose there were a supermassive, superdense hollow sphere. Inside of the sphere, would time move more slowly relative to outside? Would objects inside the sphere be contracted relative to outside? I did this calculation once about the center of a neutron star. (Of course, it isn't hollow.) Time was contracted significantly, but it depended strongly on the radius of the star. This is not know precisely so it wasn't possible to make an accurate estimate. But as you can see, if there is contraction then this will tend to build on itself. More contraction => more density => more gravity => more contraction and so on until everything collapses.