Why does cesium lose an electron more easily than lithium?

Cesium loses an electron more easily than lithium primarily because of the difference in their atomic size and the resulting ionization energy required to remove a valence electron.12

In the periodic table, atoms generally increase in size as you move down a group. Because cesium has a much larger atomic radius than lithium, its single valence electron is located significantly farther from the positively charged nucleus. Consequently, this electron experiences a weaker electrostatic attraction to the nucleus and is less strongly held.213

The ionization energy—the minimum energy required to remove an electron—decreases as you move down the alkali metal group. Since cesium has a lower ionization energy than lithium, it requires less energy to remove its valence electron, making it more chemically reactive and more likely to lose that electron to form a cation.32