It's really hard to pin down when you think about this. Western countries may be easier actually. In PNG though, coastal villagers tend to be lean and wiry but the highlanders tend to be shorter, but solid. I don't mean fat, I mean very solid. Centuries of evolution I guess in colder climates at higher altitudes. I'd also say there is no such thing as a fat kid in PNG (if there are, they're rare) however in the cities, there are certainly plenty of grossly overweight adults, which I'd assume is not genetic.
Apart from weighing every passenger for each flight, there will never be an accurate way of working with weight. From what I see of this study, it's simply attempting to ascertain if the "standard" weight assumptions are close to supportable for Finnair purposes.