Well, I didn't quite understand what you meant by "some areas being smaller than others", but I assume you are referring to "suuralueet > tilastoalueet > pienalueet", where pienalueet are part of tilastoalueet which are part of suuralueet. This is noted here: (https://www.jyvaskyla.fi/kaavoitus/kartat-ja-paikkatiedot/karttatuotteet/aluejakoaineistot), and it also is shown whenever the city releases statistics for the areas. There used to be one publication with the population of each pienalue, but it was made with Microsoft PowerBI and cannot be opened with archive.org. 

However, these can be seen in the numeric codes of the areas. In the case of Kuokkala:
suuralue - 05 Kuokkala
tilastoalue - 0528 Pohjois-Kuokkala
pienalue - 052801 Kuokkalan keskusta
where each division adds two more digits to the identifier. 

Aside from kartta.jkl.fi, I don't think there is any official site that shows the boundaries.