I wouldn't worry about it. You're young, you might still fill out some. I know a guy who was in a play when he was 19 where he was playing a 14 year old. The play won a bunch of awards, and a year later they had to do the play again at a theater festival, but the guy didn't fit in the costumes anymore because he had filled out so much. He really didn't look believable as a 14 year old anymore. My fiance also used to be very thin, but he filled out a lot in his late teens and early 20s, and now he's a pretty big guy. Of course, I think the few years he worked in construction and demolition helped a lot.
Anyways, the if the guitar is pretty, they'll spend their time looking at that rather than how it compares to your body size. And if you're good at playing it, no one will care what you look like.
Besides, chicks dig skinny guys. Especially ones that are musicians.