Hmm, are you tall? If you're tall & skinny, thats pretty typical because tall guys have a harder time putting on weight (unless they're slobs). A possible response might be to say to the whole set "Hell, I can probably out-eat the whole lot of you at once and still not gain a pound!" Any salad-eating girl is going to be jealous, and the dudes will be nervously covering up their love handles...
Also, would you say that you're a "fit" skinny, or a somewhat unhealthy-looking skinny? Even if you're not a huge sports guy, you could DHV by saying something like, "yeah, well, I do a TON of kung fu/running/basketball, so I burn calories like a motherfucker."
You could also say that you're modelling your game after Shaggy from Scooby Doo, because he's the man! I'm not tall & skinny, but everyone loves my Shaggy impression
