I don't think Somalis/Horners have innately narrower jaws. It's variable. Let's compare one-for-one of models Horner vs. West African and MENA models to test your little theory out. The only thing madows and dheegocase have on average are rounder faces (but that doesn't include models as they are thinner than the general population).Yeah, a narrow chest, the men also have it too, it's one of our more noticeable traits. Somalis and other Horners - on average - do have smaller jaws compared to say, West Africans or Arabs. The forehead is over exaggerated to an extent though, Somalis don't have huge foreheads like people make it seem, it's just slightly more pronounced than others.
Horner Models
West African Decent and MENA Models