Well, the universe does something with extremely small but extremely heavy objects, unless you think that merely creating that situation will cause the universe to cease to exist.

You don’t even need that to understand the tension between QM and GR:

What is the gravitational field of a particle in a superposition of two different locations? What about when the superposition collapses? Does the gravity field instantly change shape, faster than light?

The consensus right now is this is so hard to measure we’ll basically never know the answer from just observations. Maybe having a gravitational influence on something at all, collapses the superposition? Maybe if you put the particles in a large enough configuration it’s impossible to maintain superposition? Maybe there’s enough background noise in our particular universe to make such a measurement permanently impossible, and we get by on a technicality? Nobody knows.

Now consider that the density of an atomic nucleus is oddly similar to the density of a black hole. And this was the path Einstein was following. Too bad you need computers to study it because of all the differential equations.