The first factor depends on the volume of the atom and thus the atomic radius, which increases going down the group; thus, the volume of an alkali metal atom increases going down the group. The mass of an alkali metal atom also increases going down the group.
The initial element is contingent upon the size of the atom and consequently the atomic radius, which enlarges descending the group; hence, the size of an alkali metal atom enlarges descending the group. The mass of an alkali metal atom also enlarges descending the group.