The atomic size increases from top to bottom in a group because new shells are being added as we go down the group.
Why does the atomic radius increase down a group?
In general, for the main group elements of the periodic table: The atomic radius increases down a group, from top to bottom, of the Periodic Table. Atomic radius decreases across a Period, from left to right, of the Periodic Table. As you go down a Group in the Periodic Table from top to bottom, the number of energy levels or electron shells increases so the atomic radius of the elements increases.