Small asteroids are not as affected by gravity as larger ones. This means gravity does not help shape the smaller asteroids into a spherical shape, like it does other larger asteroids.
Large asteroids and bodies such as moons and planets are under a great gravitational force. This helps to form them into their spherical or circular shapes. Things like mountains and valleys on planets may not be spherical, but may become more round or even smaller, the longer they are affected by gravity.
Gravity's force can be clearly seen on some large asteroids. Ceres and Vesta, for example, are beginning to take on a more round shape thanks to the effects of gravity.