Let’s do a quick refresher on typical “stellar” black holes (via National Geographic). When a star consumes hydrogen, helium, etc., its gas pressure pushes outwards. It also exudes all the beautiful energy that life gives us. Meanwhile, a star’s gravity pulls everything inward. When the outward pressure finally wins, a star will boom and we get a big explosion – a supernova. When gravity wins, the whole thing collapses and we get an infinitely dense central point (a black hole) with enough gravity to capture the fastest thing in the cosmos, light.
But for all their mass and gravity, black holes aren’t too big. The small black hole V723 Monoceros (“unicorn”) has a mass three times greater than our sun (according to Astrobites), but is only 17.2 kilometers (about 10.5 miles), as Gigazine notes. According to Sci-News, the gravity of this black hole is strong enough to distort the shape of a nearby red giant star and create a bulge on its side.
Black holes like this one can grow over time and have no upper size limit. To do this, they either have to suck up the energy of a nearby star (like the black hole M33 X-7, per Nature) or collide with other black holes (like the black hole GW190521, as the Max Planck Institute for Gravitational Physics shows) on YouTube ). However, this takes an incredibly long time – so long that giant supermassive black holes simply shouldn’t exist.