It doesn't. At least not the way that science fiction movies sometimes portray black holes. A dense stellar core implodes into a black hole. This usually happens, these days, at the center of a massive star as it undergoes supernova. The core of the star is so heavy that it collapses under its own weight and there is no force to counter this gravitational collapse. Once the outer edge of the star's core passes through the so-called 'event horizon' for an object with the core's mass, it becomes a black hole and as seen by outside observers THERE IS NO FURTHER CHANGE IN THE SIZE OF THE BLACK HOLE. It just sits there for billions of years doing nothing. Black holes do not 'suck'. Instead, it is the misfortune of matter in the wrong trajectories which causes matter to fall past the black hole's event horizon and get permanently trapped. For some kinds of black holes, the kinds that rotate, there is an effect just outside of them called the Lorentz-Tirring Effect or 'frame dragging' which prevents any object from settling into a stable orbit around the black hole. In some sense, the black hole 'sucks' matter out of these trajectories and traps it.