It's more to do with size, as smaller bodies will loose their heat to space more rapidly than larger bodies. However, even this assumption has led to some surprises. The moon, for example, was once thought to be solid - far too small to retain any heat today - but recently was found to have a fluid outer core and mantle with partial melt. Ultimately, as magma rises due to its buoyancy relative to the surrounding mantle rock, the moon may actually become volcanically active in the future if the partial melt continues to rise upwards through its mantle before it solidifies.