Why has Mercury continued to shrink as it cooled?

Mercury has continued to shrink primarily due to the ongoing cooling of its interior. As the planet’s mantle cools and potentially as portions of its once-molten iron core freeze, the interior volume decreases, causing the planet's cold, rigid surface crust to buckle and contract.12

This global contraction is evidenced by hundreds of lobate scarps—cliffs formed by faulting where sections of the crust were thrust over adjacent terrain. Because these features are found across the surface and some have altered relatively young impact craters, it is clear that the planet has undergone significant shrinkage over time. The presence of many small, geologically young scarps suggests that this cooling and contraction process has not yet finished, meaning Mercury is still slowly shrinking today.12