Why are cesium atoms so big?
Cesium has a large valence shell and a relatively low effective nuclear charge. The valence shell (the outer ‘peel’ of the atom) is largest in atoms at the bottom of the periodic table. A low nuclear charge means that electrons can wander further, on average, from the nucleus. Nuclear charge increases going left to right across table rows, so the largest atoms should be found on the left edge of the table.
