Many languages treat a final CVC syllable as though it were light, while otherwise treating CVC syllables as heavy. This CVC weight asymmetry has been dealt with in several formal ways, such as with final consonant extrametricality (e.g., Hayes 1981) and as a catalectic final syllable (Kiparsky 1991). This paper argues for a phonetic basis for the asymmetry, showing that word-final CVC rhymes have a smaller proportional increase in duration over a final CV rhyme than non-final CVC rhymes have over non-final CV rhymes. Proportional increase is known to be an important aspect of human perception of differences (Weber's Law). The difference in increase between final and non-final rhymes, when compared to a short rhyme in the same position of the word, is due to the presence of phonetic final lengthening. It is thus argued that no special formal mechanism is needed to capture the behavior of final syllables, because they behave exactly as expected, given a proportional increase threshold for weight.