Big objects retain heat better than small objects do.
Most of Earth's internal heat is generated by four long-lived radioisotopes--potassium 40, thorium 232, uranium 235 and uranium 238--that release energy over billions of years as they decay into stable isotopes. Earth's large size (about 12,740 kilometers across) ensures that this heat is lost relatively slowly, which explains why our planet still has a molten outer core and volcanic eruptions at its surface. Smaller bodies, however, have a larger ratio of surface area to volume, allowing them to cool down faster by radiating their heat into space. Earth's moon, for example, is only about one fourth the size of Earth, so it loses heat much more quickly. As a result, major lunar eruptions of basalt, the most common volcanic rock, ceased nearly three billion years ago.