The theory of size-sound symbolism holds that certain phonetic and/or acoustic features of linguistic sounds are able to symbolise the smallness or largeness of different objects. For instance, it has been established that palatal consonants tend to be perceived as “smaller” than non-palatal ones. Similarly, a number of experiments have demonstrated that high front vowels tend to be associated with the notion “small in size,” and open back vowels are more suitable for representing objects which are big. This tendency is supposed to be universal across languages, but some exceptions to the rule have also been noted (cf. Ultan 1978; Diffloth 1994). This article is a continuation of the research described in Stolarski (2011), which showed that palatality is a very important factor in size-sound symbolism in Polish. The palatal /ɲ/ was perceived as “smaller” than the non-palatal /n/ in over 95% of cases. The present paper deals with a similar tendency among vowels. Pairs of artificial words containing natural Polish diminutive suffixes were presented to a representative group of respondents whose task was to choose the “smaller” forms. The obtained results clearly indicate that the high front /i/ is interpreted as much more likely to indicate a diminutive than the open back /a/. The conclusion which emerges from this research is that size-sound symbolism plays a significant role within the Polish diminutive system.