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 primary factor is influenced by the volume of the atom, which is linked to the atomic radius, and this expands as you go down the group; thus, the volume of an alkali metal atom expands as you go down the group. The mass of an alkali metal atom also expands as you go down the group.