The more massive an object, the more gravity it exerts. Mass has nothing to do with volume, except in that other forces (electromagnetism, nuclear forces)resist an object being made too small. If the mass, and gravity, are enough to overpower these other forces, everything collapses to a point (called a black hole), and at that scale, quantum effects become important. Just how that works is, well, the first person who figures that out is guaranteed to get the Nobel Prize!

It turns out that recent experiments discovered that gravity can have a quantum effect. Gravity is a very week force, particularly at small distance scales, making it extremely difficult to see its quantum effects. The larger things are the more subtle the quantum effects are.

Even small molecules are almost too large to see quantum effects clearly. So how did physicists measure the quantum effects for gravity? Ultracold neutrons are very slow-moving, uncharged particles. Physicists isolated these neutrons from the effects of the other three fundamental forces (electromagnetism, and strong and weak nuclear forces) and followed the progress of hundreds of these ultracold neutrons falling from the top of a detector to the bottom.

The team of physicists discovered that the particles only existed at certain heights! They didn't move continuously, but rather jumped from one height to another, just as quantum theory predicts! This research was done just over 2 years ago, and the implications are still being explored.