Because a remodeling of the feeding apparatus means they do not eat during the metamorphosis, the metamorphosis has to go faster the smaller the individual is, so it happens at an early stage when the larvae are still small.
bɪkˈʌz ɐ ɹᵻmˈɑːdəlɪŋ ʌvðə fˈiːdɪŋ ˌæpɚɹˈæɾəs mˈiːnz ðeɪ duːnˌɑːt ˈiːt dˈʊɹɹɪŋ ðə mˌɛɾəmˈoːɹfəsˌɪs, ðə mˌɛɾəmˈoːɹfəsˌɪs hɐz tə ɡˌoʊ fˈæstɚ ðə smˈɔːlɚ ðɪ ˌɪndᵻvˈɪdʒuːəl ɪz, sˌoʊ ɪt hˈæpənz æɾ ɐn ˈɜːli stˈeɪdʒ wɛn ðə lˈɑːɹviː ɑːɹ stˈɪl smˈɔːl.