As the atoms increase in size going down the group (because their atomic radius increases), the nuclei of the ions move further away from the delocalised electrons and hence the metallic bond becomes weaker so that the metal can more easily melt and boil, thus lowering the melting and boiling points.
æz ðɪ ˈæɾəmz ˈɪŋkɹiːs ɪn sˈaɪz ɡˌoʊɪŋ dˌaʊn ðə ɡɹˈuːp bɪkˈʌz ðɛɹ ɐtˈɑːmɪk ɹˈeɪdɪəs ˈɪŋkɹiːsᵻz, ðə nˈuːklɪi ʌvðɪ ˈaɪɑːnz mˈuːv fˈɜːðɚɹ ɐwˈeɪ fɹʌmðə dᵻlˈɑːkəlˌaɪzd ᵻlˈɛktɹɑːnz ænd hˈɛns ðə mətˈælɪk bˈɑːnd bɪkˌʌmz wˈiːkɚ sˌoʊ ðætðə mˈɛɾəl kæn mˈoːɹ ˈiːzili mˈɛlt ænd bˈɔɪl, ðˈʌs lˈoʊɚɹɪŋ ðə mˈɛltɪŋ ænd bˈɔɪlɪŋ pˈɔɪnts.