MHM go for it. I work for Virgin and I'm not exactly what you would call skinny! Any good airline will look past this and see your personality and base their decision on how well you perform at an interview. I flew with a girl a few weeks ago who was quite a big girl and she had only been with the company a month so clearly she hadn't 'piled it on' in such a short time.
Good luck and don't worry about your weight. I would also avoid any airline that ask for your weight on the application form. How very dare they
I'm sure the EU has now made it discrimitory to do this!