The point I was making was that a 'diminutive' denotes not just the affix itself, but the resulting word, which is bigger than the word was before the affix was affixed.

Yes, affixes are one way that (inflectional) languages go about created new words from old ones. In linguistics this is usually called derivational morphology to keep it separate from desinences which usually indicate case, number, gender, and other grammatical categories. The diminutive refers not to the affix or the word, but usually the referent.

An infix?

There are circumfixes, too.

Ceci n'est pas un seing.