A common question about moons like Phobos is why are they so strangely shaped - they aren't spherical like other moons. The answer is that with planets, they are spherical because gravity pulls from the center of the planet, and makes all of the edges of the planet equal, creating a sphere. Phobos does not have the mass to become spherical under it's own gravity - if it was bigger, then it would likely be more shape like an ellipse.