Answer: A new shell is added at every step down a group, so the outermost electrons lie farther from the nucleus.
Going down a group the atomic number increases and the extra electrons enter a new, higher shell. Although the nuclear charge also increases, the number of shells increases at the same time and the outer electrons are well shielded from the nucleus by the inner electrons. The distance of the outermost shell from the nucleus therefore increases, so the atomic radius increases down a group.