Here's why:

* Atomic Size Trend: Atomic size generally increases as you move down a group in the periodic table. This is because you're adding electron shells, making the atoms larger.

* Alkaline Earth Metals: The alkaline earth metals are in Group 2 of the periodic table.

* Radium's Position: Radium is at the bottom of Group 2, meaning it has the most electron shells and the largest atomic radius among the alkaline earth metals.