Why are alkali-metal atoms relatively large?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

Alkali-metal atoms are relatively large because they possess only a single, weakly bound electron located in a large outer s-type orbital.1

As you move through the periodic table, the addition of an electron to an alkali metal atom results in that electron occupying a larger orbital—such as the 4s orbital for potassium—rather than a smaller, inner orbital. This configuration, characterized by a single valence electron that is not strongly held, contributes to the overall large atomic size and the resulting low density of these metals.1