They only grow bigger. Stephen Hawking has theorized that black holes can evaporate and get smaller. I haven't seen his formulas, and I haven't read about any evidence for such a phenomenon, so it hasn't been proven. Anyways, the Andromeda galaxy is due to collide with our Milky Way galaxy in about 5 billion years. Even if the central supermassive black holes miss each other, they could each capture hundreds of millions of stars and become extremely bloated. Subsequent galactic collisions would hypothetically become more frequent, according to my simulation, and the central black hole growth will accelerate.

On a larger scale, if our universe is also a black hole within a larger universe, then it's growth will accelerate, but then abruptly stop growing, having acquired the entire mass of the parent universe. There will be no big crunch, no big rip, only growth from the inside. I suppose I could call my hypothesis a theory based on the experimental evidence of the accelerating expansion of the universe. But that's just one piece of evidence and I'm not satisfied with just that.

There is still the question of what medium supports the largest black hole? Turtles? We used to think the earth was flat and supported by turtles. Now cosmologists think the universe is flat. LOL. I think that event horizons are not physical things. They exist only as a mathematical consequence of more basic things, such as fundamental particles. Therefore nothing exists outside the largest event horizon. There is only an outside to our universe's event horizon because it has not yet enveloped the maximum mass allowed by the "field" that supports matter and energy. So it grows until it reaches the total mass of the quantum field. I think the largest possible event horizon has no "outside", just as it has no physical substance. It is an emergent mathematical phenomenon.

If one had the ability to affect quantum fields, one could affect the whole universe. But what supports the quantum field? That's got to be a mystery. Spooky too. The math seems to be rather non-intuitive. Truely alien.