I definitely agree that hurting someone's feelings is both unnecessary and hard, and I would try to avoid it. Funny enough, by the end of the flight, the interactions between this guy and myself and people around us made me realize he was quite obnoxious, but I wouldn't have known that at the outset.

Also: full flight. No other seat out there.

My feeling about hurting someone's feelings is tempered by this issue: they knew before they bought one ticket that they didn't fit in one seat. So isn't this their responsibility? I wouldn't pass judgement or tell them to diet, but I would want the (already small) seat I paid for. I'm 6'2" and 160 - I know I fit in one seat just fine (at least width-wise).