While you can fit nearly four Moons across the Earth's equator, human brains routinely confuse diameter with volume. If the Earth were hollow, it would take 49 Moons to fill it up.
When astronomical sources cite the Moon's size, they refer to its diameter. The Earth is 7,917 miles (12,742 kilometers) across at the equator, while the Moon is 2,159 miles (3,474 kilometers) across. The ratio is 27.3%, making the Moon slightly larger than one-quarter of the Earth's width. The "one-third" figure is a common misremembrance of this metric, though the Moon is unusually large relative to its host planet compared to the rest of the solar system.
However, when you look at a visual model of the Moon next to the Earth, you are not just comparing their widths. You are comparing three-dimensional spheres. Because of the square-cube law, when the diameter of an object increases, its volume scales up by the cube of that increase.
If a sphere has one-quarter the diameter of a larger sphere, it has about one-sixteenth the surface area and just one-sixty-fourth the volume. Applying the exact mathematical ratio between the two bodies reveals that the Moon holds only 2% of the Earth's volume.
This geometric reality distorts visual intuition. When told an object is a quarter of the size of another, a person often expects an object that takes up 25% of the visual bulk. Instead, they are presented with a sphere that contains just 2% of the total volume. This causes the smaller body to look disproportionately tiny—like placing a standard globe next to an exercise ball—even when the underlying math of their diameters is perfectly accurate.