The size and mass of celestial bodies are determined by a combination of factors, including their composition and formation processes. In the case of the Moon and Earth, their contrasting sizes and masses can be attributed to a specific event known as the Giant Impact Hypothesis.
According to this widely accepted hypothesis, the Moon was formed around 4.5 billion years ago when a Mars-sized object, often referred to as Theia, collided with the early Earth. This massive impact resulted in the ejection of a considerable amount of debris from both bodies, which eventually coalesced to form the Moon.
During the collision, a significant amount of Earth's mass was transferred to the Moon. However, the Moon ended up with a much smaller mass compared to Earth due to several factors. One key factor is that the debris that formed the Moon was primarily derived from the outer layers of the Earth and Theia, rather than their dense cores. The ejected material from the collision primarily consisted of lighter elements and compounds, which contributed to the Moon's lower overall mass.
Additionally, the collision was an extremely energetic event, leading to the ejection of a substantial amount of material into space. Some of this material escaped the gravitational pull of both bodies, further reducing the mass of the Moon.
The Moon's smaller mass and size in relation to Earth are a consequence of the specific conditions and dynamics of the Giant Impact event, which resulted in a redistribution of mass and material between the two bodies.