The entropy of formation of Na-Cs liquid alloys, Delta S, is nearly ideal. This is surprising considering that the ratio between the pure metal volumes is about 3.0. The authors show by means of a density functional calculation that the ratio between the effective volumes in the liquid alloy changes to nearly 1.5. This ratio, used in conjunction with Flory's formula for the entropy of mixing, allows quite accurate reproduction of the ideal behaviour of Delta S. They also show that this feature of the ratios of atomic volumes is not exclusive to Na-Cs but is common to all the family of binary liquid alkali alloys.