The Nintendo character Kirby's name in Japanese is 「星のカービィ」, pronounced "Hoshi no Kābī". For the カ, a ー is used for extending the vowel sound. For the ビ, though, a small イ is used.
I thought the small vowels were used for modifying the vowel in an existent sound without extending it, for example in ティ, to make "ti", or in ヴィ, to make "vi". With ビ, however, that is of course not necessary, as the vowel is already "i". In hiragana, normal vowels are used for extension, but those are 1) not small, and 2) not used in katakana.
So why is this? And how does it work? Under what grammatical rule does the small vowel extend the sound, and why is ー used for カ and not for ビ (which I think it normally would, such as in 「ビール」)?