Definition from Wiktionary, the free dictionary
-illa (masculine counterpart -illo)
- Added to feminine nouns to denote a diminutive form.
Usage notes 
- If the noun has a final vowel (usually -a), it is dropped before adding -illa.
- In most cases, -illa is used simply to indicate a small or endeared thing, without changing the basic meaning of the noun; however, in some cases, it is used to effect a greater change in meaning, such as bombilla (“light-bulb”), from bomba (“bomb”).
- Different nouns tend to prefer different diminutive suffixes (see synonyms below), though some nouns accept multiple diminutives and there is often regional or personal variation.
- For masculine nouns ending in “-a”, there is some alternation between the gender-reflecting diminutive -illo and the ending-reflecting diminutive -illa. When the latter is used, -ill- may be considered an infix.