Here's why:
* Atomic Size Trends: As you move down a group (column) in the periodic table, atomic size increases. This is because you're adding electron shells, which makes the atoms larger.
* Group: All of these elements belong to Group 2 (alkaline earth metals) of the periodic table.
* Order: Magnesium is at the top of the group, followed by Calcium, Strontium, and then Barium.
Therefore, Magnesium is the smallest, followed by Calcium, Strontium, and finally Barium.
