Demure,
no more deserts, no more walking, no more famines........genetics can be over-ridden

Metabolism probably has a role to do with that too.......becuase arid dry environments where there aren't many resources encourage a slower metabolism. Somalis naturally don't have large muscle mass to support as that would have needed more food to support.
But in general, even though ALL the ones abroad may not be ultra skinny, there are still a whole lot of them that are skinny, despite eating a lot.........
even abroad, Somalis are still known for their slender figures.