Fats in fish ain't a real thing at least not ones I eat, they more lean then chicken and higher protein. Also can't site anything but I heard multiple times with introduciton of Mcdonalds and other fatty american places in Japan cup sizes among other things tend to be on the rise, could be just they sold out and got more boob jobs or just cows, and grains like wheat help you bulk up more while fish, and game meat seem to keep you lighter...
That is my two sense on it, also evolution wise they have no need at all to be taller or fatter up north, as they have furs to stay warm, and being small means wasting less energy. Also every thing you do closer to the artict poles is harder and harder. The closer you get the more calories you burn. I mean people in the south and what not should get less harsh winters, and area is more tamed so to speak while north every thing is harder, just your daily chores like cutting firewood and what not stuff southerns do as well is more labor intense with the climate, the treck through deeper snow, and what not to fact the timber to cut, and very act of being outside and active in general means you burn more trying to stay warm as well.
I mean I live in Arizona, but I use to go up to Flagstaff in our local mountains from the desert valley. Its like 4 hour or so drive, and every thing is harder up there, I vacation so its not bad, but I got wear a ski suit and what not as I go there to ski, and most northern tribes should be using skis, cross country mind you way more then southern tribes. I mean that is my least favorite type. I got family in northen woods of Oregon as well not even close to climates in finland I bet, but even in the summer its completly diffrent experince, but in the winter oh no they burn twice if not more the caloreis I do in south.
Its a bit of extreme comparison a desert to the Northern woods but point is I think its largely to climate and well just breeding. I mean they are not hooking up with the big tall southerners so they likely to produce small kids just like they are, just like their way of life dictates they would be a little skinnier, yet stronger. I mean I got big mountain man bloodline on my dad's side but that is from France, with rumors a little norwidgen in there some where, but for Frances cold weather woods and what not my ancestors were well adapted as being in middle of Europe they would be fighting more and have more civilization there. I mean during this age my ancestors were probably working fields for a lord, or caring a sword for one. In either case them big like we are and can handle snowy winters in the woods, yet still hold the line in a battle we very well suited.
But the north tribes here I can't see them having to many enemies or lords demanding things from them. I mean who is gonna go all the way up there to seize their ice covered soil, and pile of fish when they can get that from other places easier. I makes perfect sense they were some of the last tamed as only people there for religious conversion really get much out of "taming" the wilds of Finland. That and fur trade... I just don't see the need for the northern tribes to be big, net alone support them getting big, they eat more lean meats then the rest, and we all know grains do make you a bit on fat side