Hyperion is the largest of Saturn's irregular, nonspherical moons. Hyperion's mean radius is 83.9 miles (135 km), but since Hyperion is rather potato-shaped, its shape can be described in terms of its diameter along its three axes: 255 x 163 x 137 miles (410 x 260 x 220 km, respectively). Considering its odd shape, Hyperion is probably a remnant of a larger moon that was destroyed by a major impact.

Hyperion's density is slightly more than half that of water. This could be due to water ice with gaps (porosity) of more than 40 percent. Also, lighter materials, such as frozen methane or carbon dioxide, could make up part of Hyperion. This is consistent with the concept of Hyperion accreting from a number of smaller ice and rock bodies, but not having enough gravity to compact them. Thus, Hyperion might be similar to a large rubble pile.