As we move down the group from $\mathrm{Li}$ to $\mathrm{Cs}$, both the atomic size and atomic mass increase. But the increase in atomic mass more than compensates the bigger atomic size. As a result, the densities of alkali metals increase from $\mathrm{Li}$ to $\mathrm{Cs}$. Potassium is, however, lighter than sodium.

Hence, the increasing order of density is as given below:

$\mathrm{Li}< \mathrm{K}<\mathrm{Na}<\mathrm{Rb}<\mathrm{Cs}$