As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has an almost linear relationship with the size (molecular weight) of the molecule.
æz ðə bˈɔɪlɪŋ pˈɔɪnt ʌv ˈælkeɪnz ɪz pɹaɪmˈɛɹᵻli dɪtˈɜːmɪnd baɪ wˈeɪt, ɪt ʃˌʊd nˌɑːt biː ɐ sɚpɹˈaɪz ðætðə bˈɔɪlɪŋ pˈɔɪnt hɐz ɐn ˈɔːlmoʊst lˈɪnɪɹ ɹᵻlˈeɪʃənʃˌɪp wɪððə sˈaɪz məlˈɛkjʊlɚ wˈeɪt ʌvðə mˈɑːlɪkjˌuːl.