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.
ðə fˈɜːst fˈæktɚ dᵻpˈɛndz ɔnðə vˈɑːljuːm ʌvðɪ ˈæɾəm ænd ðˈʌs ðɪ ɐtˈɑːmɪk ɹˈeɪdɪəs, wˌɪtʃ ɪŋkɹˈiːsᵻz ɡˌoʊɪŋ dˌaʊn ðə ɡɹˈuːp; ðˈʌs, ðə vˈɑːljuːm əvən ˈælkəlˌaɪ mˈɛɾəl ˈæɾəm ˈɪŋkɹiːsᵻz ɡˌoʊɪŋ dˌaʊn ðə ɡɹˈuːp.