in some creeks and river systems larger specimens are very hard to find for a multitude of reasons . in some cases its about water volume and or the lack of . it is a well known fact fish in a small aquarium wont grow as large as fish in a large aquarium .
there may also be about a bunch of other factors involved , but to be sure on the real shallow small creeks you find many if any large panfish .
it could be many things that cause them to run small , but the good news is most fish dont have a per say finite maximum growth potential and if they live long enough they might attain larger size .
keep in mind fighting current most of your life will also keep you slim and trim