Hugh Gellatts said:
while we are on the subject, why are samoans big?
Same as with Africans. Pacific islands can only handle so many people so it was customary for pioneers to head out in groups to find new territory where they could be the boss. They did this in frickin' row boats. If you observe the Pacific ocean you will no doubt notice it is damn big and the islands are really, really far apart. These journeys weeded out the skinny phenotype, that's a long time to be exposed to extremes of temperature on the open ocean.
"The results support the hypothesis that exposure would have been a major problem for Neolithic voyagers in the Pacific, and the likelihood of strong direction selection for a muscular phenotype. The survival proportions for all groups were less than anticipated. For example survival proportions in summer for males of intermediate physique across a range of latitudes from 12oS to 25oS are 62, 56, 19, 5 and zero. For all groups, whatever their range of survival, there is a rapid decline in survival beyond 15o latitude, and this association of survival with latitude was, in statistical terms, highly significant. For any group there is an approximately 5% decline in survival for each degree movement away from ten degrees of latitude, with an average male/female difference in survival of 5% in summer and 3% in winter, to the advantage of the males.
The inference is that the impressive Polynesian muscularity has evolved not for locomotor purposes but as a metabolic heat source. The particular muscle fibre type predominantly involved in shivering (an isometric contraction) is Type IIb, or fast twitch6,7,8. We suggest that people of Polynesian ancestry are likely to show a genetically-determined preponderance of this muscle fibre type."
http://www.monash.edu.au/APJCN/Vol4/Num4/44p354.htm#top
Samoans are also reputed to be the descendants of people who also survived famine on the way across, for some reason the fishing was poor, so only the fat ones made it.
It's possible that the selective breeding practiced on Africans in America made some difference but I don't know. Selective breeding takes many, many generations. It's not as the same as with marijuana where a generation lasts 60 days (ahem). Some of the Europeans, particularly the Irish in the 1840's had similar mortality rates on the Atlantic journey with no athletic windfall for the gene pool. I'd say if West Africans had an American diet for a couple of generations the results would be the same, or maybe better, as 90% of African-Americans are of mixed African and European ancestry. Look at the way the Japanese shot up after world war II when they imported a different diet.