How did Mercury get smaller?

Answer 1

When Mercury's iron core cooled off it made Mercury shrunk half a mile and when it shrunk it made cliffs and from space it looks like its wrinkly its from the cooling episode.

~Unknown Teller~

Answer 2

Mercury's small size is believed to have been caused by a high-velocity impact early in its formation, which stripped away much of its outer layers. This impact likely occurred during a period of intense bombardment in the early solar system. Additionally, Mercury's proximity to the Sun and gravitational interactions with other planets may have also contributed to its compact size.