Kokanee size

Why are the kokanee in Montana so much smaller than the ones here in Washington?  Been noticing that from the pics from various people and boards... wondering if anyone knows why that is or what causes it?

Not sure how big they are in Montana but Lake Merwin produces 13" pretty regularly. Maybe they bulk up to outrun the Tiger Muskies!

Most of our kok pics come off of American Lake. That place just doesn't make small Kokanee. Even the 13" fish are too fat to wrap a hand around. Plenty of scrawny skinny koks in our other lakes

I read one of the kokanee fishing books , can't remember the one, but I believe the size depends on the number of kokes in the lake and the kind of feed in the particular lake. Just from reading forums I have gathered that it can vary year to year also.