My roommate jokes that it has more to do with my genetics/heritage than anything, but I stick by my drinking-water-explanation.
Both are true. Ever heard of alcohol flush response? It's an affliction that hits Native Americans, some East Asians and Australian Aborigines when they drink, and its existence explains why those groups are notoriously susceptible to the effects of alcohol. Quite simply, some cultures boiled water or relied on fresh foods in order to have a sanitary diet while most Western cultures went with yeast, fermentation and grains. Quite simply, Europeans tend to have far more of the enzymes handle alcohols in their system than those other groups do. It's essentially the same phenomena that leads to a high proportion of lactose intolerance in Asians. It really is quite possible that your ethnicity could be allowing you to process alcohol more quickly. For example, my family did some genetic testing recently to get a better idea of what exactly our ancestry is, and I'm Norwegian, Irish and predominantly Spanish (I have a lot of family hailing from Mexico from as far back as the hacienda system, but apparently they didn't mix with the natives much). I'm a small man, so I can get drunk quickly, but I also tend to bounce back extremely quickly, and if I match each drink with water, I can drink for stupidly long periods of time and be up by 8.
As for the hangover severity goes, the difference between you and Nobo probably has more to do with hydration than metabolism or weight. Muscle retains water better than fat does, so as a big muscular guy, it makes sense that Nobo can put back quite a few without approaching his tolerance. Unfortunately, it also makes sense that he'd be in for a world of hurt once he crosses his threshold, because muscle holds all that water for a reason, and once he's finally drained he's no better off than the rest of us.
