The vowel-size relationship has been repeatedly reported: the vowels /a/ and /i/ elicit bigger/smaller images respectively. Previous studies reporting this relationship have required participants to make explicit decisions about the meaning of the target words including these vowels. In the present study, we attempted to re-examine the vowel-size relationship in two experiments using speeded classification tasks. The results of Experiment 1 indicate that participants associated the vowels with a bigger/smaller image even when they were not motivated to pronounce the vowels during the task. The results of Experiment 2 indicate that the proprioception of the absolute size of the mouth may not contribute to the vowel-size relationship. The process underpinning the vowel-size relationship is discussed.