Why are people from Nordic countries so physically strong?
VICE did a pretty great short documentary on why Iceland, despite having a population less than one-third of a million, produces so many "strongmen".
The reason primarily being the harsh climate (temperatures routinely reach as low as -30 degrees Celsius) and the fact that the country is isolated from any other major landmass. Here is a map:
Being near the Arctic, it experiences months of darkness. And until a few years ago, entertainment options were limited. A few young, focused individuals turned to powerlifting, and soon the craze caught on, especially after the success of Jón Páll Sigmarsson in the late 80s.
I suspect that the impression of "Nordic strength" comes from the prominence of Northern Europeans among the world's strongmen, rather than the average citizens of those countries tending to be stronger than the average citizens of other countries. Looking at average height/size (which I think is a reasonable proxy for physical strength), the average Finn or Dane isn't particularly larger than the average Greek, Spaniard, or Hungarian, so I doubt that he or she has a substantially higher deadlift max. When you're asking this, are you thinking of a burly Icelander tossing kegs around?
The underlying question then would be "why are Northern Europeans so prominent in strongman competitions?"
As some other folks have mentioned, good health and nutrition probably play a big role in this. You can't be skipping too many breakfasts if you want to look like Magnús Ver Magnússon. So Scandinavians have a bit of an advantage over, say, North Koreans. However, while nutrition is necessary for strength, it is by no means sufficient: there are plenty of countries with equally good nutrition that don't "overproduce" strongmen.
I suspect that Nordic strongmen, like Jamaican sprinters, are products of similar phenomena: a relative lack of other top-level professional sports to draw away potential talent. The physical ability and raw size that make someone a great strongman are also quite useful in American football, basketball, rugby, and other sports. In places where those sports are prominent and remunerative, they will tend to draw that physical talent. The sort of American who would likely excel at strength competitions might also have the chance to go to college on a football scholarship, so he'd likely do that instead, not to mention the millions he could make if he goes pro.
This isn't to say that Scandinavian players don't play those other sports, just that they seem to not be subjected to the same structural "tracking" to high level professional leagues as people in other nations seem to be. If we look at the List of foreign NBA players, we see that Croatia (pop. 4.2 million) has had more NBA players than Norway, Denmark, Sweden, and Iceland combined. On a pure population level, wouldn't think that the athletes of those nations are any less likely to be tall or physically gifted than Croatians, so something else is going on.
Think of Usain Bolt. He's the best sprinter in the world, but with his size, speed, and hand-eye coordination (he's also a great cricket player), an American-born Bolt would have been playing American football from an early age, and we'd now know him as a particularly fast NFL player rather than an Olympian. American football wasn't as much of an option in Jamaica, however, while sprinting was.
I suspect that something similar is going on in places like Norway and Sweden. The strongest folks there are less likely to play professional rugby, basketball, or American football than people in other countries (those sports are less popular, less developed youth programs, etc.), and so would be likely to end up as strength athletes, especially once their nation develops a "strong" tradition in the sport. So the reason there seem to be so many Nordic strongmen is because they aren't playing football.
Hafthór Júlíus Björnsson is a great case example of this sort of talent overlap. He was formerly a professional basketball player in Iceland, but transitioned quite well into the world of strength. The potential was always there, but when he was playing basketball it didn't make much sense for him to bulk up to 400 lbs. Once he hurt his knee, it did. Brian Shaw is another example, a successful college basketball player who became the world's strongest man.
