The suffix -kin seems to have a somewhat variable distribution across speakers. For some it appears to be a productive suffix that means "little" or "baby": if you have a wug, a wugkin would be a small or juvenile wug. For other speakers, like myself, -kin is not productive, and in fact I barely noticed the fact that it has any sort of smallish connotation until this was pointed out to me. How can this be the case for a single language like English? Because we're all exposed to differing dialects, registers, and ultimately, types and amounts of data. Someone growing up as an only child on a rural farm during the 13th century is certain to be exposed to less language that a child from a family of fourteen growing up in modern day New York. And even leaving aside such differences, we all hear slightly different corpora from slightly different speakers as we grow up. (This is, of course, the source of language change, especially when speaker communities have little interaction with each other.)
Perception also plays an important role. Words ending in -kin have a fairly fixed distribution in English. Despite what I said in the previous paragraph, I'd be surprised if words containing it varied significantly across regions, registers, or dialects (and of course this could be empirically tested). When I was acquiring English, I perceived the suffix as being rare enough that I did not generalize it to other forms, unlike very common suffixes like -er for 'one who...', e.g., farmer 'one who farms'. It also may be due to the fact that if you ask me for a word ending in -kin, the first one I go to is "munchkin", which is certainly not "a small munch". So why is this interesting? I find it interesting because it shows that even when exposed to (essentially) the same data, speakers will or won't generalize different patterns. Patterns that are pervasive in a language (75% seems to be the magic number in Artificial Language Learning tasks) are for the most part generalized by all speakers: everyone who speakers English has the suffix -er and is willing to use it on novel forms. Patterns that derive from a previously productive affix but are now opaque are almost never generalized: the prefix "with-" as in "withstand" indicating "against" is, I would wager, never used with novel forms. The interesting patterns are those that occur in a small but semi-regular subdomain of English, as with the Germanic "strong verb" pattern of strike/struck generalizing to sneak/snuck, or the case of -kin. (Firefox won't recognize "snuck", but it gets millions of ghits, including an entry on Dictionary.com.) Speakers have to decide when confronted with such data whether these cases are simply a handful of random exceptions, or if there is some regular pattern that applies to only a small subset of lexemes.