The difference in size and mass between the Moon and Earth can be explained by their different compositions and formation processes.
The Moon's smaller size compared to Earth is due to its formation through a giant impact event called the "Giant Impact Hypothesis." According to this hypothesis, around 4.5 billion years ago, a Mars-sized object, often referred to as Theia, collided with the early Earth. The impact caused a significant portion of Earth's outer layers to be ejected into space, and the debris eventually coalesced to form the Moon. This process resulted in the Moon having a smaller size compared to Earth.
The difference in mass between the Moon and Earth is primarily due to their different compositions. Earth has a dense core made up mostly of iron and nickel, which contributes significantly to its overall mass. In contrast, the Moon has a much smaller core and a higher concentration of lighter elements such as silicates. The Moon's composition is similar to Earth's mantle and crust but lacks a substantial iron-nickel core. As a result, the Moon has a much lower mass compared to Earth, despite its relatively large size.
The specific values of 25% size and 1.2% mass you mentioned may vary slightly depending on the exact measurements, but the overall trend of the Moon being smaller in size and significantly lighter in mass than Earth remains true.
It's worth noting that the size and mass of celestial bodies are determined by a combination of factors, including their formation processes, composition, and subsequent geological activities. The Moon's formation through a giant impact event and its different composition compared to Earth are the primary reasons for the observed differences in size and mass between the two bodies.