This is how I am.  I look pretty thin (BMI of 22.6, but muscular), and people are always joking about why I need to workout.  Ummm...because that's how I stay this way.  If I were like them...well...I'd be huge like them.  I get pissy when I gain the extra 3-5 pounds I always gain in the offseason (not letting it happen this year).  How do these things not realize that they're gaining that every month?!